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TooTallNate merged 2 commits into from
Mar 16, 2026
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Fix prepareStep system message lost when messages is also returned #1389

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9426b01
Fix prepareStep system message lost when messages is also returned
TooTallNate Mar 14, 2026
93cf09e
Merge branch 'main' into fix/prepare-step-system-message-ordering
VaguelySerious Mar 16, 2026
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5 changes: 5 additions & 0 deletions .changeset/fix-prepare-step-system-ordering.md
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned

@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)

@pranaygppranaygpMar 15, 2026

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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)

maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion packages/ai/src/agent/stream-text-iterator.test.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
LanguageModelV3Prompt,
LanguageModelV3ToolCall,
LanguageModelV3ToolResult,
LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
});
});
});

describe('prepareStep system and messages ordering', () => {
it('should apply system message when prepareStep returns only system', async () => {
const mockWritable = createMockWritable();
const mockModel = vi.fn();

let capturedPrompt: LanguageModelV3Prompt | undefined;

vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
capturedPrompt = prompt;
return {
toolCalls: [],
finish: { finishReason: 'stop' },
step: createMockStepResult({ finishReason: 'stop' }),
};
});

const iterator = streamTextIterator({
prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
tools: {} as ToolSet,
writable: mockWritable,
model: mockModel as any,
prepareStep: () => ({
system: 'You are a helpful assistant.',
}),
});

await iterator.next();

expect(capturedPrompt).toBeDefined();
expect(capturedPrompt![0]).toEqual({
role: 'system',
content: 'You are a helpful assistant.',
});
expect(capturedPrompt![1]).toEqual({
role: 'user',
content: [{ type: 'text', text: 'hello' }],
});
});

it('should preserve system message when prepareStep returns both system and messages', async () => {
const mockWritable = createMockWritable();
const mockModel = vi.fn();

let capturedPrompt: LanguageModelV3Prompt | undefined;

vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
capturedPrompt = prompt;
return {
toolCalls: [],
finish: { finishReason: 'stop' },
step: createMockStepResult({ finishReason: 'stop' }),
};
});

// prepareStep returns both system and messages — system should NOT be lost
const customMessages: LanguageModelV3Prompt = [
{
role: 'user',
content: [{ type: 'text', text: 'modified message' }],
},
];

const iterator = streamTextIterator({
prompt: [
{ role: 'user', content: [{ type: 'text', text: 'original' }] },
],
tools: {} as ToolSet,
writable: mockWritable,
model: mockModel as any,
prepareStep: () => ({
system: 'Dynamic system prompt.',
messages: customMessages,
}),
});

await iterator.next();

expect(capturedPrompt).toBeDefined();
// System message should be prepended to the replaced messages
expect(capturedPrompt!).toHaveLength(2);
expect(capturedPrompt![0]).toEqual({
role: 'system',
content: 'Dynamic system prompt.',
});
expect(capturedPrompt![1]).toEqual({
role: 'user',
content: [{ type: 'text', text: 'modified message' }],
});
});

it('should replace existing system message when messages already contains one', async () => {
const mockWritable = createMockWritable();
const mockModel = vi.fn();

let capturedPrompt: LanguageModelV3Prompt | undefined;

vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
capturedPrompt = prompt;
return {
toolCalls: [],
finish: { finishReason: 'stop' },
step: createMockStepResult({ finishReason: 'stop' }),
};
});

// Messages already include a system message — prepareStep's system should replace it
const customMessages: LanguageModelV3Prompt = [
{ role: 'system', content: 'Old system prompt.' },
{ role: 'user', content: [{ type: 'text', text: 'hello' }] },
];

const iterator = streamTextIterator({
prompt: [
{ role: 'user', content: [{ type: 'text', text: 'original' }] },
],
tools: {} as ToolSet,
writable: mockWritable,
model: mockModel as any,
prepareStep: () => ({
system: 'New system prompt.',
messages: customMessages,
}),
});

await iterator.next();

expect(capturedPrompt).toBeDefined();
expect(capturedPrompt!).toHaveLength(2);
expect(capturedPrompt![0]).toEqual({
role: 'system',
content: 'New system prompt.',
});
expect(capturedPrompt![1]).toEqual({
role: 'user',
content: [{ type: 'text', text: 'hello' }],
});
});

it('should update system message on subsequent steps', async () => {
const mockWritable = createMockWritable();
const mockModel = vi.fn();

const capturedPrompts: LanguageModelV3Prompt[] = [];

const toolCall: LanguageModelV3ToolCall = {
type: 'tool-call',
toolCallId: 'call-1',
toolName: 'testTool',
input: '{}',
};

vi.mocked(doStreamStep)
.mockImplementationOnce(async (prompt) => {
capturedPrompts.push([...prompt]);
return {
toolCalls: [toolCall],
finish: { finishReason: 'tool-calls' },
step: createMockStepResult({ finishReason: 'tool-calls' }),
};
})
.mockImplementationOnce(async (prompt) => {
capturedPrompts.push([...prompt]);
return {
toolCalls: [],
finish: { finishReason: 'stop' },
step: createMockStepResult({ finishReason: 'stop' }),
};
});

const iterator = streamTextIterator({
prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
tools: {
testTool: {
description: 'Test',
execute: async () => ({ ok: true }),
},
} as ToolSet,
writable: mockWritable,
model: mockModel as any,
prepareStep: ({ stepNumber: sn }) => ({
system: `System prompt v${sn}`,
}),
});

// First step
await iterator.next();

// Provide tool results
const toolResults: LanguageModelV3ToolResultPart[] = [
{
type: 'tool-result',
toolCallId: 'call-1',
toolName: 'testTool',
output: { type: 'text', value: '{"ok":true}' },
},
];

// Second step
await iterator.next(toolResults);

expect(capturedPrompts).toHaveLength(2);
// First step should have system v0
expect(capturedPrompts[0][0]).toEqual({
role: 'system',
content: 'System prompt v0',
});
// Second step should have system v1
expect(capturedPrompts[1][0]).toEqual({
role: 'system',
content: 'System prompt v1',
});
});
});
});
10 changes: 6 additions & 4 deletions packages/ai/src/agent/stream-text-iterator.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
if (prepareResult.model !== undefined) {
currentModel = prepareResult.model;
}
if (prepareResult.messages !== undefined) {
conversationPrompt = [...prepareResult.messages];
}
if (prepareResult.system !== undefined) {
// Update or prepend system message in the conversation prompt
// Update or prepend system message in the conversation prompt.
// Applied AFTER messages override so the system message isn't
// lost when messages replaces the prompt.
if (
conversationPrompt.length > 0 &&
conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
});
}
}
Comment on lines 151 to 174
if (prepareResult.messages !== undefined) {
conversationPrompt = [...prepareResult.messages];
}
if (prepareResult.experimental_context !== undefined) {
currentContext = prepareResult.experimental_context;
}
Expand Down
Loading
, 'i'); if (__m === '*' || __re.test(location.href)) { // Add copy buttons to all 
 blocks
(function() {
function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;
codeBlock.parentElement.setAttribute('data-copy-added', 'true');
var btn = document.createElement('button');
btn.textContent = 'Copy';
btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';
btn.onmouseover = function() { this.style.opacity = '1'; };
btn.onmouseout = function() { this.style.opacity = '0.7'; };
btn.onclick = function() {
navigator.clipboard.writeText(codeBlock.textContent).then(function() {
btn.textContent = 'Copied!';
setTimeout(function() { btn.textContent = 'Copy'; }, 1500);
});
};
codeBlock.parentElement.style.position = 'relative';
codeBlock.parentElement.appendChild(btn);
});
}
addCopyButtons();
// Re-run on dynamic content
var observer = new MutationObserver(addCopyButtons);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
Skip to content
Fix prepareStep system message lost when messages is also returned#1389

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 Merged

TooTallNate
merged 2 commits into

from

Mar 16, 2026
 Merged


Fix prepareStep system message lost when messages is also returned
#1389
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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Force GitHub README to respect dark mode
(function() {
var style = document.createElement('style');
style.textContent = '
.markdown-body {
color-scheme: dark light;
}
.markdown-body pre { background: #161b22 !important; }
.markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }
.markdown-body table th, .markdown-body table td { border-color: #30363d !important; }
.markdown-body img { background: #0d1117; }
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';
document.head.appendChild(style);
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}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
Skip to content
Fix prepareStep system message lost when messages is also returned#1389

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Already on GitHub?
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 Merged

TooTallNate
merged 2 commits into

from

Mar 16, 2026
 Merged


Fix prepareStep system message lost when messages is also returned
#1389
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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
var terms = [];
if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {
var url = new URL(ref);
var q = url.searchParams.get('q') || url.searchParams.get('p');
if (q) {
terms = q.split(/\s+/).filter(function(t) { return t.length > 2; });
}
}
if (terms.length === 0) return;
var style = document.createElement('style');
style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';
document.head.appendChild(style);
function highlight(node) {
if (node.nodeType === 3) { // text node
var text = node.textContent;
var found = false;
terms.forEach(function(term) {
var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\]\\]/g, '\\') + ')', 'gi');
if (regex.test(text)) {
found = true;
var frag = document.createDocumentFragment();
var parts = text.split(regex);
parts.forEach(function(part, i) {
if (i % 2 === 0) {
frag.appendChild(document.createTextNode(part));
} else {
var span = document.createElement('span');
span.className = 'userscript-highlight';
span.textContent = part;
frag.appendChild(span);
}
});
node.parentNode.replaceChild(frag, node);
}
});
} else if (node.nodeType === 1 && node.childNodes) { // element
var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];
if (!skipTags.includes(node.tagName)) {
Array.from(node.childNodes).forEach(highlight);
}
}
}
highlight(document.body);
// Re-highlight on dynamic content
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1 || node.nodeType === 3) highlight(node);
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
Skip to content
Fix prepareStep system message lost when messages is also returned#1389

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 Merged

TooTallNate
merged 2 commits into

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Mar 16, 2026
 Merged


Fix prepareStep system message lost when messages is also returned
#1389
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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
function cleanUrl(url) {
try {
var u = new URL(url, window.location.origin);
var changed = false;
trackingParams.forEach(function(p) {
if (u.searchParams.has(p)) {
u.searchParams.delete(p);
changed = true;
}
});
return changed ? u.toString() : url;
} catch (e) {
return url;
}
}
function cleanLinks() {
document.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
cleanLinks();
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1) {
if (node.tagName === 'A') cleanLinks();
node.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
Skip to content
Fix prepareStep system message lost when messages is also returned#1389

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Merged

TooTallNate
merged 2 commits into

from

Mar 16, 2026
 Merged


Fix prepareStep system message lost when messages is also returned
#1389
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Commits

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2 commits


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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
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Fix prepareStep system message lost when messages is also returned#1389

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TooTallNate
merged 2 commits into

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Mar 16, 2026
 Merged


Fix prepareStep system message lost when messages is also returned
#1389
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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
el.style.position = 'static';
el.style.top = 'auto';
el.style.zIndex = 'auto';
}
});
}
unstick();
var observer = new MutationObserver(unstick);
observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });
})();
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
Skip to content
Fix prepareStep system message lost when messages is also returned#1389

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 Merged

TooTallNate
merged 2 commits into

from

Mar 16, 2026
 Merged


Fix prepareStep system message lost when messages is also returned
#1389
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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

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Contributor
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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
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Fix prepareStep system message lost when messages is also returned by TooTallNate · Pull Request #1389 · vercel/workflow · GitHub
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TooTallNate
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Mar 16, 2026
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Fix prepareStep system message lost when messages is also returned
#1389
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9426b01

Fix prepareStep system message lost when messages is also returned


TooTallNate Mar 14, 2026
93cf09e

Merge branch 'main' into fix/prepare-step-system-message-ordering


VaguelySerious Mar 16, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-prepare-step-system-ordering.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@workflow/ai': patch
---

Fix `prepareStep` system message being discarded when `messages` is also returned
@pranaygppranaygpMar 15, 2026

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can we add an e2e test for this to prevent regressions @TooTallNate? since we now have the DurableAgent e2e test infra as of this week :)
@pranaygppranaygpMar 15, 2026

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also when relevant/possible, we should update the DurableAgent chat app to use the feature - for example when adding/fixing provider tools and client side tools, I would've been good to add that feature to the app as an easy way to try it and see if it actually works and the DX for it (the app didn't exist at the time and the friction to do a tarball and test with flight booking app is a bit annoying)
maybe this time we should have something in the DurableAgent test app that uses prepareStep?

217 changes: 216 additions & 1 deletion
packages/ai/src/agent/stream-text-iterator.test.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -10,9 +10,10 @@ import type {
 LanguageModelV3Prompt,
 LanguageModelV3ToolCall,
 LanguageModelV3ToolResult,
 LanguageModelV3ToolResultPart,
} from '@ai-sdk/provider';
import type { StepResult, ToolSet, UIMessageChunk } from 'ai';
import { describe, expect, it, vi, beforeEach } from 'vitest';
import { beforeEach, describe, expect, it, vi } from 'vitest';

// Mock doStreamStep
vi.mock('./do-stream-step.js', () => ({
Expand DownExpand Up@@ -651,4 +652,218 @@ describe('streamTextIterator', () => {
 });
 });
 });

 describe('prepareStep system and messages ordering', () => {
 it('should apply system message when prepareStep returns only system', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'You are a helpful assistant.',
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'You are a helpful assistant.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should preserve system message when prepareStep returns both system and messages', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // prepareStep returns both system and messages — system should NOT be lost
 const customMessages: LanguageModelV3Prompt = [
 {
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'Dynamic system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 // System message should be prepended to the replaced messages
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'Dynamic system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'modified message' }],
 });
 });

 it('should replace existing system message when messages already contains one', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 let capturedPrompt: LanguageModelV3Prompt | undefined;

 vi.mocked(doStreamStep).mockImplementationOnce(async (prompt) => {
 capturedPrompt = prompt;
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 // Messages already include a system message — prepareStep's system should replace it
 const customMessages: LanguageModelV3Prompt = [
 { role: 'system', content: 'Old system prompt.' },
 { role: 'user', content: [{ type: 'text', text: 'hello' }] },
 ];

 const iterator = streamTextIterator({
 prompt: [
 { role: 'user', content: [{ type: 'text', text: 'original' }] },
 ],
 tools: {} as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: () => ({
 system: 'New system prompt.',
 messages: customMessages,
 }),
 });

 await iterator.next();

 expect(capturedPrompt).toBeDefined();
 expect(capturedPrompt!).toHaveLength(2);
 expect(capturedPrompt![0]).toEqual({
 role: 'system',
 content: 'New system prompt.',
 });
 expect(capturedPrompt![1]).toEqual({
 role: 'user',
 content: [{ type: 'text', text: 'hello' }],
 });
 });

 it('should update system message on subsequent steps', async () => {
 const mockWritable = createMockWritable();
 const mockModel = vi.fn();

 const capturedPrompts: LanguageModelV3Prompt[] = [];

 const toolCall: LanguageModelV3ToolCall = {
 type: 'tool-call',
 toolCallId: 'call-1',
 toolName: 'testTool',
 input: '{}',
 };

 vi.mocked(doStreamStep)
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [toolCall],
 finish: { finishReason: 'tool-calls' },
 step: createMockStepResult({ finishReason: 'tool-calls' }),
 };
 })
 .mockImplementationOnce(async (prompt) => {
 capturedPrompts.push([...prompt]);
 return {
 toolCalls: [],
 finish: { finishReason: 'stop' },
 step: createMockStepResult({ finishReason: 'stop' }),
 };
 });

 const iterator = streamTextIterator({
 prompt: [{ role: 'user', content: [{ type: 'text', text: 'hello' }] }],
 tools: {
 testTool: {
 description: 'Test',
 execute: async () => ({ ok: true }),
 },
 } as ToolSet,
 writable: mockWritable,
 model: mockModel as any,
 prepareStep: ({ stepNumber: sn }) => ({
 system: `System prompt v${sn}`,
 }),
 });

 // First step
 await iterator.next();

 // Provide tool results
 const toolResults: LanguageModelV3ToolResultPart[] = [
 {
 type: 'tool-result',
 toolCallId: 'call-1',
 toolName: 'testTool',
 output: { type: 'text', value: '{"ok":true}' },
 },
 ];

 // Second step
 await iterator.next(toolResults);

 expect(capturedPrompts).toHaveLength(2);
 // First step should have system v0
 expect(capturedPrompts[0][0]).toEqual({
 role: 'system',
 content: 'System prompt v0',
 });
 // Second step should have system v1
 expect(capturedPrompts[1][0]).toEqual({
 role: 'system',
 content: 'System prompt v1',
 });
 });
 });
});

10 changes: 6 additions & 4 deletions
packages/ai/src/agent/stream-text-iterator.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -148,8 +148,13 @@ export async function* streamTextIterator({
 if (prepareResult.model !== undefined) {
 currentModel = prepareResult.model;
 }
 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.system !== undefined) {
 // Update or prepend system message in the conversation prompt
 // Update or prepend system message in the conversation prompt.
 // Applied AFTER messages override so the system message isn't
 // lost when messages replaces the prompt.
 if (
 conversationPrompt.length > 0 &&
 conversationPrompt[0].role === 'system'
Expand All@@ -167,9 +172,6 @@ export async function* streamTextIterator({
 });
 }
 }

Comment on lines

151
 to 
174

 if (prepareResult.messages !== undefined) {
 conversationPrompt = [...prepareResult.messages];
 }
 if (prepareResult.experimental_context !== undefined) {
 currentContext = prepareResult.experimental_context;
 }
Expand Down
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