fix: stop on detected model fallback via stream detection, not a model whitelist#281

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Merged
Tryanks merged 1 commit into from
Aug 25, 2026
Merged

fix: stop on detected model fallback via stream detection, not a model whitelist #281

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1 change: 0 additions & 1 deletion crates/agent/examples/probe.rs
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Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
let opts = SessionOptions {
cwd,
model,
abort_on_model_fallback: true,
resume: None,
fork: false,
binary_path: None,
Expand Down
214 changes: 126 additions & 88 deletions crates/agent/src/claude.rs
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Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
// Resolve model-scoped launch options from the persisted selections
// (effort/context/fast/thinking are launch-time only; mid-session changes
// ride the resume-restart machinery).
let launch = ClaudeLaunchOptions::resolve(
opts.model.as_deref(),
&opts.option_selections,
opts.abort_on_model_fallback,
);
let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
let base_permission_mode = permission_mode_flag(opts.approval_mode);
let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
// Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
let base = model.split('[').next().unwrap_or(model);
let primary = if base.contains("fable") {
"fable"
} else {
base
};
let mut allow = vec![primary.to_string()];
for extra in ["sonnet", "haiku"] {
if !allow.iter().any(|m| m == extra) {
allow.push(extra.to_string());
}
}
allow
}

fn launch_settings_json(
model: Option<&str>,
abort_on_model_fallback: bool,
thinking: Option<bool>,
fast_mode: bool,
ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
if ultracode {
settings.insert("ultracode".into(), json!(true));
}
if abort_on_model_fallback && let Some(model) = model {
settings.insert(
"availableModels".into(),
json!(fallback_guard_models(model)),
);
settings.insert("switchModelsOnFlag".into(), json!(false));
}
(!settings.is_empty())
.then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
fn resolve(
model: Option<&str>,
selections: &[OptionSelection],
abort_on_model_fallback: bool,
) -> Self {
fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
let spec = model.and_then(model_spec);
let raw_effort = selection_str(selections, "reasoningEffort");
let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
None
};

let settings_json = launch_settings_json(
model,
abort_on_model_fallback,
thinking,
fast_mode,
ultracode,
);
let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

ClaudeLaunchOptions {
model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
(expected.contains("fable") && served.contains("opus"))
|| (served.contains("opus-4-8")
&& expected.contains("opus")
&& !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
session_started: bool,
current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
last_served_model: Option<String>,
/// Model selected for this session, used when Claude reports a synthetic refusal message.
expected_model: Option<String>,
/// Whether a served-model mismatch was already reported for the active turn.
fallback_detected: bool,
/// Latest structured stop reason for the active turn.
stop_reason: Option<String>,
/// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
native_rewind,
last_served_model: None,
expected_model,
fallback_detected: false,
stop_reason: None,
pending_refusal_category: None,
warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
self.current_turn_id = Some(id.clone());
self.awaiting_turn_checkpoint = self.native_rewind;
self.exit_plan_captured = false;
self.fallback_detected = false;
self.stop_reason = None;
self.pending_refusal_category = None;
self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
events
}

fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
let (Some(expected), Some(actual)) = (
msg.get("original_model").and_then(Value::as_str),
msg.get("fallback_model").and_then(Value::as_str),
) else {
log::debug!("claude: ignoring malformed model_refusal_fallback");
return Vec::new();
};
self.fallback_detected = true;
vec![AgentEvent::ModelFallbackDetected {
expected: expected.to_owned(),
actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
.and_then(Value::as_str)
.map(ClassifierCategory::parse);
}
if let Some(model) = message.get("model").and_then(Value::as_str)
&& self.last_served_model.as_deref() != Some(model)
{
self.last_served_model = Some(model.to_owned());
out.push(AgentEvent::ServedModel {
model: model.to_owned(),
reason: None,
});
if let Some(model) = message.get("model").and_then(Value::as_str) {
if !self.fallback_detected
&& let Some(expected) = self.expected_model.as_deref()
{
let normalized_expected = expected
.split('[')
.next()
.unwrap_or(expected)
.to_ascii_lowercase();
let normalized_served = model
.split('[')
.next()
.unwrap_or(model)
.to_ascii_lowercase();
if served_model_is_fallback(&normalized_expected, &normalized_served) {
self.fallback_detected = true;
out.push(AgentEvent::ModelFallbackDetected {
expected: expected.to_owned(),
actual: model.to_owned(),
category: None,
checkpoint_id: None,
parent_tool_use_id: msg
.get("parent_tool_use_id")
.and_then(Value::as_str)
.map(str::to_owned),
});
}
}
if self.last_served_model.as_deref() != Some(model) {
self.last_served_model = Some(model.to_owned());
out.push(AgentEvent::ServedModel {
model: model.to_owned(),
reason: None,
});
}
}
out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
);
}

#[test]
fn served_model_fallback_family_rule() {
let cases = [
("claude-fable-5", "claude-opus-4-8", true),
("claude-fable-5", "claude-opus-5", true),
("claude-opus-5", "claude-opus-4-8", true),
("claude-opus-4-8", "claude-opus-4-8", false),
("claude-fable-5", "claude-fable-5", false),
("claude-fable-5", "claude-sonnet-4-5", false),
("claude-fable-5", "claude-haiku-4-5", false),
("anything", "<synthetic>", false),
];
for (expected, served, is_fallback) in cases {
assert_eq!(
served_model_is_fallback(expected, served),
is_fallback,
"expected={expected}, served={served}"
);
}

let expected = "claude-opus-5[1m]";
let normalized_expected = expected.split('[').next().unwrap_or(expected);
assert!(served_model_is_fallback(
normalized_expected,
"claude-opus-4-8"
));
}

#[test]
fn assistant_model_mismatch_emits_one_fallback_per_turn() {
let mut mapper = Mapper::new_configured(
false,
InteractionMode::Build,
"default",
"default".into(),
ApprovalMode::Supervised,
false,
Some("claude-fable-5".into()),
);
mapper.start_turn();

let first = feed(
&mut mapper,
r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
);
assert_eq!(
first
.iter()
.filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
.count(),
1
);
assert!(first.iter().any(|event| matches!(
event,
AgentEvent::ModelFallbackDetected {
expected,
actual,
category: None,
checkpoint_id: None,
parent_tool_use_id: None,
} if expected == "claude-fable-5"
&& actual == "claude-opus-4-8"
)));

let second = feed(
&mut mapper,
r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
);
assert!(
!second
.iter()
.any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
);
}

#[test]
fn classifier_refusal_result_emits_turn_blocked() {
let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
value: json!(true),
},
],
false,
);
assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
value: json!("1m"),
},
],
false,
);
assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
id: "thinking".into(),
value: json!(true),
}],
false,
);
let settings: Value =
serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
assert_eq!(settings["alwaysThinkingEnabled"], true);
}

#[test]
fn fallback_guard_settings_merge_with_launch_settings() {
let parse = |model, thinking, fast_mode, ultracode| -> Value {
serde_json::from_str(
launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
.as_deref()
.unwrap(),
)
.unwrap()
};

let fable = parse("claude-fable-5", Some(true), true, true);
assert_eq!(
fable["availableModels"],
json!(["fable", "sonnet", "haiku"])
);
assert_eq!(fable["switchModelsOnFlag"], false);
assert_eq!(fable["alwaysThinkingEnabled"], true);
assert_eq!(fable["fastMode"], true);
assert_eq!(fable["ultracode"], true);

let opus = parse("claude-opus-5", None, false, false);
assert_eq!(
opus["availableModels"],
json!(["claude-opus-5", "sonnet", "haiku"])
);

let opus_1m = parse("claude-opus-5[1m]", None, false, false);
assert_eq!(
opus_1m["availableModels"],
json!(["claude-opus-5", "sonnet", "haiku"])
);
}

#[test]
fn todo_write_maps_to_plan_updated() {
let mut m = Mapper::new();
Expand Down
1 change: 0 additions & 1 deletion crates/agent/src/lib.rs
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Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
pub cwd: PathBuf,
/// Provider-native model id; `None` = provider default.
pub model: Option<String>,
pub abort_on_model_fallback: bool,
pub resume: Option<ResumeCursor>,
/// Fork the resumed provider session instead of continuing it in place.
/// This is meaningful only when `resume` is present.
Expand Down
3 changes: 2 additions & 1 deletion crates/runtime/src/app/events.rs
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Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
.filter(|active| active.meta.id == session_id)
.is_some_and(|active| {
active.queue.clear();
active.timeline.mark_idle();
active.shutdown_to_idle();
true
});
if is_active {
self.interrupt(cx);
self.emit_domain(
Topic::SessionStatus {
session_id: session_id.to_owned(),
Expand Down
1 change: 0 additions & 1 deletion crates/runtime/src/app/providers.rs
View file
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Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
SessionOptions {
cwd: meta.cwd.clone(),
model: meta.model.clone(),
abort_on_model_fallback: settings.abort_on_model_fallback,
resume: meta.resume_cursor.clone(),
fork: meta.pending_fork,
binary_path: provider_settings.binary_path.clone(),
Expand Down
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fix: stop on detected model fallback via stream detection, not a model whitelist#281

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 Merged

Tryanks
merged 1 commit into

from

Aug 25, 2026
 Merged


fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

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Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

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Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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fix: stop on detected model fallback via stream detection, not a model whitelist#281

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merged 1 commit into

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Aug 25, 2026
 Merged


fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
fix: stop on detected model fallback via stream detection, not a model whitelist#281

New issue

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privacy statement. We’ll occasionally send you account related emails.

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to your account

 Merged

Tryanks
merged 1 commit into

from

Aug 25, 2026
 Merged


fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

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fix: stop on detected model fallback via stream detection, not a model whitelist#281

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Aug 25, 2026
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fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
fix: stop on detected model fallback via stream detection, not a model whitelist#281

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merged 1 commit into

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Aug 25, 2026
 Merged


fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

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Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
fix: stop on detected model fallback via stream detection, not a model whitelist#281

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 Merged

Tryanks
merged 1 commit into

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Aug 25, 2026
 Merged


fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

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fix: stop on detected model fallback via stream detection, not a model whitelist#281

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Aug 25, 2026
 Merged


fix: stop on detected model fallback via stream detection, not a model whitelist
#281
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1 change: 0 additions & 1 deletion
crates/agent/examples/probe.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -245,7 +245,6 @@ async fn run_probe(
 let opts = SessionOptions {
 cwd,
 model,
 abort_on_model_fallback: true,
 resume: None,
 fork: false,
 binary_path: None,
Expand Down

214 changes: 126 additions & 88 deletions
crates/agent/src/claude.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -109,11 +109,7 @@ pub async fn start(opts: SessionOptions) -> Result<SessionHandle, AgentError> {
 // Resolve model-scoped launch options from the persisted selections
 // (effort/context/fast/thinking are launch-time only; mid-session changes
 // ride the resume-restart machinery).
 let launch = ClaudeLaunchOptions::resolve(
 opts.model.as_deref(),
 &opts.option_selections,
 opts.abort_on_model_fallback,
 );
 let launch = ClaudeLaunchOptions::resolve(opts.model.as_deref(), &opts.option_selections);
 let base_permission_mode = permission_mode_flag(opts.approval_mode);
 let launch_permission_mode = initial_permission_mode(opts.approval_mode, opts.interaction_mode);
 // Launch arguments are deliberately appended last, so the tracker must
Expand DownExpand Up@@ -317,25 +313,7 @@ struct ClaudeLaunchOptions {
 ultrathink: bool,
}

fn fallback_guard_models(model: &str) -> Vec<String> {
 let base = model.split('[').next().unwrap_or(model);
 let primary = if base.contains("fable") {
 "fable"
 } else {
 base
 };
 let mut allow = vec![primary.to_string()];
 for extra in ["sonnet", "haiku"] {
 if !allow.iter().any(|m| m == extra) {
 allow.push(extra.to_string());
 }
 }
 allow
}

fn launch_settings_json(
 model: Option<&str>,
 abort_on_model_fallback: bool,
 thinking: Option<bool>,
 fast_mode: bool,
 ultracode: bool,
Expand All@@ -350,23 +328,12 @@ fn launch_settings_json(
 if ultracode {
 settings.insert("ultracode".into(), json!(true));
 }
 if abort_on_model_fallback && let Some(model) = model {
 settings.insert(
 "availableModels".into(),
 json!(fallback_guard_models(model)),
 );
 settings.insert("switchModelsOnFlag".into(), json!(false));
 }
 (!settings.is_empty())
 .then(|| serde_json::to_string(&Value::Object(settings)).unwrap_or_default())
}

impl ClaudeLaunchOptions {
 fn resolve(
 model: Option<&str>,
 selections: &[OptionSelection],
 abort_on_model_fallback: bool,
 ) -> Self {
 fn resolve(model: Option<&str>, selections: &[OptionSelection]) -> Self {
 let spec = model.and_then(model_spec);
 let raw_effort = selection_str(selections, "reasoningEffort");
 let resolved_effort = resolve_claude_effort(spec.as_ref(), raw_effort.as_deref());
Expand DownExpand Up@@ -399,13 +366,7 @@ impl ClaudeLaunchOptions {
 None
 };

 let settings_json = launch_settings_json(
 model,
 abort_on_model_fallback,
 thinking,
 fast_mode,
 ultracode,
 );
 let settings_json = launch_settings_json(thinking, fast_mode, ultracode);

 ClaudeLaunchOptions {
 model_id,
Expand DownExpand Up@@ -1025,6 +986,13 @@ struct PendingRewind {
 conversation: bool,
}

fn served_model_is_fallback(expected: &str, served: &str) -> bool {
 (expected.contains("fable") && served.contains("opus"))
 || (served.contains("opus-4-8")
 && expected.contains("opus")
 && !expected.contains("opus-4-8"))
}

pub(crate) struct Mapper {
 session_started: bool,
 current_message_id: Option<String>,
Expand DownExpand Up@@ -1095,6 +1063,8 @@ pub(crate) struct Mapper {
 last_served_model: Option<String>,
 /// Model selected for this session, used when Claude reports a synthetic refusal message.
 expected_model: Option<String>,
 /// Whether a served-model mismatch was already reported for the active turn.
 fallback_detected: bool,
 /// Latest structured stop reason for the active turn.
 stop_reason: Option<String>,
 /// Classifier category captured from the active turn's structured refusal details.
Expand DownExpand Up@@ -1158,6 +1128,7 @@ impl Mapper {
 native_rewind,
 last_served_model: None,
 expected_model,
 fallback_detected: false,
 stop_reason: None,
 pending_refusal_category: None,
 warned_stop_reason: None,
Expand All@@ -1180,6 +1151,7 @@ impl Mapper {
 self.current_turn_id = Some(id.clone());
 self.awaiting_turn_checkpoint = self.native_rewind;
 self.exit_plan_captured = false;
 self.fallback_detected = false;
 self.stop_reason = None;
 self.pending_refusal_category = None;
 self.warned_stop_reason = None;
Expand DownExpand Up@@ -1464,14 +1436,15 @@ impl Mapper {
 events
 }

 fn on_model_refusal_fallback(&self, msg: &Value) -> Vec<AgentEvent> {
 fn on_model_refusal_fallback(&mut self, msg: &Value) -> Vec<AgentEvent> {
 let (Some(expected), Some(actual)) = (
 msg.get("original_model").and_then(Value::as_str),
 msg.get("fallback_model").and_then(Value::as_str),
 ) else {
 log::debug!("claude: ignoring malformed model_refusal_fallback");
 return Vec::new();
 };
 self.fallback_detected = true;
 vec![AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: actual.to_owned(),
Expand DownExpand Up@@ -1755,14 +1728,41 @@ impl Mapper {
 .and_then(Value::as_str)
 .map(ClassifierCategory::parse);
 }
 if let Some(model) = message.get("model").and_then(Value::as_str)
 && self.last_served_model.as_deref() != Some(model)
 {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 if let Some(model) = message.get("model").and_then(Value::as_str) {
 if !self.fallback_detected
 && let Some(expected) = self.expected_model.as_deref()
 {
 let normalized_expected = expected
 .split('[')
 .next()
 .unwrap_or(expected)
 .to_ascii_lowercase();
 let normalized_served = model
 .split('[')
 .next()
 .unwrap_or(model)
 .to_ascii_lowercase();
 if served_model_is_fallback(&normalized_expected, &normalized_served) {
 self.fallback_detected = true;
 out.push(AgentEvent::ModelFallbackDetected {
 expected: expected.to_owned(),
 actual: model.to_owned(),
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: msg
 .get("parent_tool_use_id")
 .and_then(Value::as_str)
 .map(str::to_owned),
 });
 }
 }
 if self.last_served_model.as_deref() != Some(model) {
 self.last_served_model = Some(model.to_owned());
 out.push(AgentEvent::ServedModel {
 model: model.to_owned(),
 reason: None,
 });
 }
 }
 out.extend(self.observe_stop_reason(message.get("stop_reason").and_then(Value::as_str)));
 let msg_id = message
Expand DownExpand Up@@ -3149,6 +3149,81 @@ mod tests {
 );
 }

 #[test]
 fn served_model_fallback_family_rule() {
 let cases = [
 ("claude-fable-5", "claude-opus-4-8", true),
 ("claude-fable-5", "claude-opus-5", true),
 ("claude-opus-5", "claude-opus-4-8", true),
 ("claude-opus-4-8", "claude-opus-4-8", false),
 ("claude-fable-5", "claude-fable-5", false),
 ("claude-fable-5", "claude-sonnet-4-5", false),
 ("claude-fable-5", "claude-haiku-4-5", false),
 ("anything", "<synthetic>", false),
 ];
 for (expected, served, is_fallback) in cases {
 assert_eq!(
 served_model_is_fallback(expected, served),
 is_fallback,
 "expected={expected}, served={served}"
 );
 }

 let expected = "claude-opus-5[1m]";
 let normalized_expected = expected.split('[').next().unwrap_or(expected);
 assert!(served_model_is_fallback(
 normalized_expected,
 "claude-opus-4-8"
 ));
 }

 #[test]
 fn assistant_model_mismatch_emits_one_fallback_per_turn() {
 let mut mapper = Mapper::new_configured(
 false,
 InteractionMode::Build,
 "default",
 "default".into(),
 ApprovalMode::Supervised,
 false,
 Some("claude-fable-5".into()),
 );
 mapper.start_turn();

 let first = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-1","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert_eq!(
 first
 .iter()
 .filter(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 .count(),
 1
 );
 assert!(first.iter().any(|event| matches!(
 event,
 AgentEvent::ModelFallbackDetected {
 expected,
 actual,
 category: None,
 checkpoint_id: None,
 parent_tool_use_id: None,
 } if expected == "claude-fable-5"
 && actual == "claude-opus-4-8"
 )));

 let second = feed(
 &mut mapper,
 r#"{"type":"assistant","message":{"id":"msg-fallback-2","model":"claude-opus-4-8","content":[]},"parent_tool_use_id":null}"#,
 );
 assert!(
 !second
 .iter()
 .any(|event| matches!(event, AgentEvent::ModelFallbackDetected { .. }))
 );
 }

 #[test]
 fn classifier_refusal_result_emits_turn_blocked() {
 let mut mapper = Mapper::new_configured(
Expand DownExpand Up@@ -3403,7 +3478,6 @@ mod tests {
 value: json!(true),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-opus-4-8"));
 assert_eq!(launch.effort.as_deref(), Some("xhigh"));
Expand All@@ -3426,7 +3500,6 @@ mod tests {
 value: json!("1m"),
 },
 ],
 false,
 );
 assert_eq!(launch.model_id.as_deref(), Some("claude-fable-5[1m]"));
 assert_eq!(launch.effort, None);
Expand All@@ -3440,47 +3513,12 @@ mod tests {
 id: "thinking".into(),
 value: json!(true),
 }],
 false,
 );
 let settings: Value =
 serde_json::from_str(launch.settings_json.as_deref().unwrap()).unwrap();
 assert_eq!(settings["alwaysThinkingEnabled"], true);
 }

 #[test]
 fn fallback_guard_settings_merge_with_launch_settings() {
 let parse = |model, thinking, fast_mode, ultracode| -> Value {
 serde_json::from_str(
 launch_settings_json(Some(model), true, thinking, fast_mode, ultracode)
 .as_deref()
 .unwrap(),
 )
 .unwrap()
 };

 let fable = parse("claude-fable-5", Some(true), true, true);
 assert_eq!(
 fable["availableModels"],
 json!(["fable", "sonnet", "haiku"])
 );
 assert_eq!(fable["switchModelsOnFlag"], false);
 assert_eq!(fable["alwaysThinkingEnabled"], true);
 assert_eq!(fable["fastMode"], true);
 assert_eq!(fable["ultracode"], true);

 let opus = parse("claude-opus-5", None, false, false);
 assert_eq!(
 opus["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );

 let opus_1m = parse("claude-opus-5[1m]", None, false, false);
 assert_eq!(
 opus_1m["availableModels"],
 json!(["claude-opus-5", "sonnet", "haiku"])
 );
 }

 #[test]
 fn todo_write_maps_to_plan_updated() {
 let mut m = Mapper::new();
Expand Down

1 change: 0 additions & 1 deletion
crates/agent/src/lib.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -393,7 +393,6 @@ pub struct SessionOptions {
 pub cwd: PathBuf,
 /// Provider-native model id; `None` = provider default.
 pub model: Option<String>,
 pub abort_on_model_fallback: bool,
 pub resume: Option<ResumeCursor>,
 /// Fork the resumed provider session instead of continuing it in place.
 /// This is meaningful only when `resume` is present.
Expand Down

3 changes: 2 additions & 1 deletion
crates/runtime/src/app/events.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -344,10 +344,11 @@ impl AppState {
 .filter(|active| active.meta.id == session_id)
 .is_some_and(|active| {
 active.queue.clear();
 active.timeline.mark_idle();
 active.shutdown_to_idle();
 true
 });
 if is_active {
 self.interrupt(cx);
 self.emit_domain(
 Topic::SessionStatus {
 session_id: session_id.to_owned(),
Expand Down

1 change: 0 additions & 1 deletion
crates/runtime/src/app/providers.rs

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -779,7 +779,6 @@ pub(super) fn session_options(
 SessionOptions {
 cwd: meta.cwd.clone(),
 model: meta.model.clone(),
 abort_on_model_fallback: settings.abort_on_model_fallback,
 resume: meta.resume_cursor.clone(),
 fork: meta.pending_fork,
 binary_path: provider_settings.binary_path.clone(),
Expand Down
Loading