diff --git a/AGENTS.md b/AGENTS.md
index ae38cb3a..36573640 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -86,6 +86,12 @@ macOS note: the vendored binary is unsigned — if Gatekeeper blocks it:
 - Run artifacts land in `~/.amico/runs/default/<runId>/` (contract: `run.toml`, `AMICODE_ITER`
   lines, `iter_*.png`, `result.toml`, `pulse.jld2`, `FINISHED`). Validate files with
   `packages/schema/launcher/amico-validate <file>`.
+- **New sessions open on opencode's read-only `plan` agent** (plan-first posture for all
+  users): `buildOpencodeConfigContent` injects `default_agent: "plan"`. The composer
+  agent-picker (or an explicit per-message `agent`, as in the e2e tests) switches to
+  build to execute. The picker is plan/build only — the pulse-designer agent shell is
+  retired (#389); the interview content lives in the compiled AGENTS.md score section,
+  visible to every agent.
 - Never commit to `main`; branch + PR.
 
 ## Changing opencode (the vendored fork)
diff --git a/packages/extension/src/opencode_config.ts b/packages/extension/src/opencode_config.ts
index 729b5988..0de4d636 100644
--- a/packages/extension/src/opencode_config.ts
+++ b/packages/extension/src/opencode_config.ts
@@ -113,9 +113,13 @@ export function resolveJuliaProject(configValue: string): string {
  *      plugin dir is a sibling of both). TODO(follow-up): extension.ts should
  *      pass this explicitly once .vsix packaging of opencode-plugin/ is
  *      verified; the default keeps existing call sites working unchanged.
- *    - `agent: {"pulse-designer": …}` — the interview agent; its prompt defers
- *      to the "Pulse-designer interview" section of the injected AGENTS.md so
- *      the interview script lives in ONE place.
+ *    - `default_agent: "plan"` — plan-first posture: new sessions open on
+ *      opencode's built-in read-only plan agent, not straight into execution.
+ *      The picker stays plan/build ONLY (roles-not-modes, #368): the interview
+ *      content lives in the compiled AGENTS.md score section (visible to every
+ *      agent), and the pulse-designer agent entry is RETIRED (#389) — it was
+ *      a four-line prompt shell over the config-root permission block, doing
+ *      nothing build did not already do.
  *    - an `external_directory` grant for the Problem-workspaces root, so the
  *      AGENT's file tools can read back system/formulation/run/event TOML the
  *      plugin wrote (the plugin's own fs writes are host-process calls and need
@@ -417,6 +421,13 @@ export function buildOpencodeConfigContent(
   const skills = skillsStageDir ? { paths: [skillsStageDir] } : undefined;
   return JSON.stringify({
     $schema: "https://opencode.ai/config.json",
+    // Plan-first posture (product default for ALL users): every new Amicode
+    // session opens on opencode's built-in, read-only `plan` agent; execution
+    // (the pulse-designer interview, solves) starts only when the user switches
+    // agents in the composer. Like everything in this blob, it deep-merges OVER
+    // the user's global config — an explicit per-message `agent` (the e2e tests,
+    // the distiller's --agent) is unaffected.
+    default_agent: "plan",
     ...(modelPin ? { model: modelPin } : {}),
     instructions: [agentsPath],
     plugin: [pluginPath],
@@ -424,15 +435,11 @@ export function buildOpencodeConfigContent(
     // Enable AI-SDK span generation ONLY behind the telemetry gate — deep-merges
     // into cfg.experimental alongside any user keys (see telemetryOpen above).
     ...(telemetryOpen ? { experimental: { openTelemetry: true } } : {}),
-    agent: {
-      "pulse-designer": {
-        description: "Guided quantum pulse design interview",
-        prompt:
-          "You are Amico's pulse-designer. Follow the 'Pulse-designer interview' section of " +
-          "the project instructions exactly: one question at a time, record each stage with " +
-          "the amicode_* tools, and use the solve workflow for launches.",
-      },
-    },
+    // No `agent` overrides: the picker is opencode's native plan/build only
+    // (#389 — the pulse-designer agent entry is retired; its prompt was a
+    // shell deferring to the compiled AGENTS.md interview section, and its
+    // permission grants were always the config-root block above). The
+    // interview runs from ANY agent via the injected instructions.
     permission: {
       bash: "allow",
       edit: "allow",
diff --git a/packages/extension/src/scores/compiler.ts b/packages/extension/src/scores/compiler.ts
index de0d5b58..935be536 100644
--- a/packages/extension/src/scores/compiler.ts
+++ b/packages/extension/src/scores/compiler.ts
@@ -4,8 +4,8 @@ import { ScoreManifest, Stage } from "./schema";
 
 // Compile a score into the injected-prompt section — "data-defined, prompt-executed"
 // (spec §6). The heading is kept EXACTLY "## Pulse-designer interview" for score #0
-// compatibility: the pulse-designer agent prompt in buildOpencodeConfigContent refers
-// to that section by name. Pure and deterministic: same score → same string.
+// compatibility: spliced section lookups (and the hardcoded fallback section in
+// AGENTS.md) match on it by name. Pure and deterministic: same score → same string.
 
 const INTERVIEW_CONTRACT = [
   "**Interview contract:** ONE question at a time — never batch. Ask, wait, record,",
diff --git a/packages/extension/test/opencode_config.test.ts b/packages/extension/test/opencode_config.test.ts
index 0e1c1fee..c5e54a13 100644
--- a/packages/extension/test/opencode_config.test.ts
+++ b/packages/extension/test/opencode_config.test.ts
@@ -89,14 +89,13 @@ describe("buildOpencodeConfigContent", () => {
     );
     expect(withStage.skills).toEqual({ paths: ["/tmp/proj/skills"] }); // absolute per-session dir (guarded set), never a library root
   });
-  it("declares the pulse-designer agent whose prompt defers to the AGENTS.md interview", () => {
+  it("retires the pulse-designer agent shell — the picker is plan/build only (#389)", () => {
     const cfg = JSON.parse(buildOpencodeConfigContent("/abs/AGENTS.md", TPL, "/home/u/.amico/runs/default"));
-    const pd = cfg.agent["pulse-designer"];
-    expect(pd.description).toBe("Guided quantum pulse design interview");
-    expect(pd.prompt).toContain("one question at a time"); // the interview protocol
-    expect(pd.prompt).toContain("'Pulse-designer interview'"); // script lives in AGENTS.md, not here
-    expect(pd.prompt).toContain("amicode_"); // record stages via the tool pack
-    expect(pd.prompt).toContain("solve workflow"); // launches stay on the bash workflow
+    expect(cfg.agent ?? {}).toEqual({}); // no custom agents: the interview lives in AGENTS.md, agent-agnostic
+  });
+  it("pins default_agent to plan (plan-first posture for new sessions)", () => {
+    const cfg = JSON.parse(buildOpencodeConfigContent("/abs/AGENTS.md", TPL, "/home/u/.amico/runs/default"));
+    expect(cfg.default_agent).toBe("plan"); // read-only open; the user switches to build to execute
   });
   it("grants external_directory on the problems root (default + $AMICODE_PROBLEMS_DIR override)", () => {
     const defGrant = join(homedir(), ".amico", "problems") + "/**";
@@ -238,15 +237,16 @@ describe.skipIf(!existsSync(OC_BIN))("opencode config injection + merge (1.17.3)
     // the user's global config SURVIVED the deep-merge:
     expect(cfg.model).toBe("anthropic/claude-sonnet-4-6"); // provider/model preserved (Q129 needs this)
     expect(cfg.permission.doom_loop).toBe("deny"); // user permission key preserved (#22)
-    // L0 pulse-designer registration survived resolution against the REAL binary.
+    // L0 registration survived resolution against the REAL binary.
     // NOTE: `debug config` IMPORTS listed plugins before printing JSON to stdout
     // (verified on 1.17.3) — so JSON.parse(out) succeeding above doubles as a
     // regression guard that amicode_tools.ts loads cleanly AND never writes to
     // stdout at module scope (its load line must stay on stderr).
     expect(cfg.plugin).toHaveLength(1);
     expect(cfg.plugin[0].endsWith(join("opencode-plugin", "amicode_tools.ts"))).toBe(true);
-    expect(cfg.agent["pulse-designer"].description).toBe("Guided quantum pulse design interview");
-    expect(cfg.agent["pulse-designer"].prompt).toContain("one question at a time");
+    // #389: the pulse-designer agent shell is retired; default is plan.
+    expect(cfg.agent?.["pulse-designer"]).toBeUndefined();
+    expect(cfg.default_agent).toBe("plan");
   });
 });
 
diff --git a/packages/extension/test/slow/interview_e2e.test.ts b/packages/extension/test/slow/interview_e2e.test.ts
index 9ce19e0c..32c5cdb7 100644
--- a/packages/extension/test/slow/interview_e2e.test.ts
+++ b/packages/extension/test/slow/interview_e2e.test.ts
@@ -121,10 +121,10 @@ afterAll(() => {
 });
 
 describe.skipIf(!existsSync(OC_BIN))("L0 registration against the real binary (creds-free)", () => {
-  it("A: pulse-designer appears in GET /agent", { timeout: 60_000 }, async () => {
+  it("A: the pulse-designer agent shell is retired — picker is plan/build", { timeout: 60_000 }, async () => {
     const s = await serve({ hermetic: true, port: 14310 });
     const agents = (await (await fetch(s.url + "/agent")).json()) as Array<{ name: string }>;
-    expect(agents.map((a) => a.name)).toContain("pulse-designer");
+    expect(agents.map((a) => a.name)).not.toContain("pulse-designer"); // retired (#389): picker is plan/build only
   });
 
   it.skipIf(!existsSync(PLUGIN))("B: amicode_tools plugin loads on session creation", { timeout: 60_000 }, async () => {
@@ -152,7 +152,7 @@ describe.skipIf(!existsSync(OC_BIN) || !hasCreds())("live interview turns (creds
       const r = await fetch(`${s.url}/session/${ses.id}/message`, {
         method: "POST",
         headers: { "content-type": "application/json" },
-        body: JSON.stringify({ agent: "pulse-designer", parts: [{ type: "text", text }] }),
+        body: JSON.stringify({ agent: "build", parts: [{ type: "text", text }] }),
       });
       expect(r.ok, `message POST ${r.status}`).toBe(true);
       const msg = (await r.json()) as {
@@ -208,7 +208,7 @@ describe.skipIf(!existsSync(OC_BIN) || !hasCreds())("live interview turns (creds
         const r = await fetch(`${s.url}/session/${ses.id}/message`, {
           method: "POST",
           headers: { "content-type": "application/json" },
-          body: JSON.stringify({ agent: "pulse-designer", parts: [{ type: "text", text }] }),
+          body: JSON.stringify({ agent: "build", parts: [{ type: "text", text }] }),
         });
         expect(r.ok, `message POST ${r.status}`).toBe(true);
         const msg = (await r.json()) as {
diff --git a/packages/extension/test/slow/scores_e2e.test.ts b/packages/extension/test/slow/scores_e2e.test.ts
index d76c6bdf..d04d5568 100644
--- a/packages/extension/test/slow/scores_e2e.test.ts
+++ b/packages/extension/test/slow/scores_e2e.test.ts
@@ -117,7 +117,7 @@ describe.skipIf(!existsSync(OC_BIN) || !hasCreds())("scores runtime live e2e (cr
         const r = await fetch(`${s.url}/session/${ses.id}/message`, {
           method: "POST",
           headers: { "content-type": "application/json" },
-          body: JSON.stringify({ agent: "pulse-designer", parts: [{ type: "text", text }] }),
+          body: JSON.stringify({ agent: "build", parts: [{ type: "text", text }] }),
         });
         expect(r.ok, `message POST ${r.status}`).toBe(true);
         const msg = (await r.json()) as {