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Fix prepareStep system message lost when messages is also returned - #1389
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🦋 Changeset detectedLatest commit: 93cf09e The changes in this PR will be included in the next version bump. This PR includes changesets to release 1 package
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🧪 E2E Test Results❌ Some tests failed Summary
❌ Failed Tests🌍 Community Worlds (55 failed)mongodb (3 failed):
redis (2 failed):
turso (50 failed):
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📊 Benchmark Results
workflow with no steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) workflow with 1 step💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) workflow with 10 sequential steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Express | Next.js (Turbopack) | Nitro workflow with 25 sequential steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Express | Next.js (Turbopack) | Nitro workflow with 50 sequential steps💻 Local Development
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🔍 Observability: Nitro | Next.js (Turbopack) | Express Promise.all with 10 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) Promise.all with 25 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Next.js (Turbopack) | Express Promise.all with 50 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Express | Next.js (Turbopack) | Nitro Promise.race with 10 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Next.js (Turbopack) | Nitro | Express Promise.race with 25 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Next.js (Turbopack) | Nitro | Express Promise.race with 50 concurrent steps💻 Local Development
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🔍 Observability: Express | Nitro | Next.js (Turbopack) Stream Benchmarks(includes TTFB metrics)workflow with stream💻 Local Development
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🔍 Observability: Next.js (Turbopack) | Nitro | Express SummaryFastest Framework by WorldWinner determined by most benchmark wins
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Pull request overview
Fixes a DurableAgent/streamTextIterator bug where a prepareStep callback returning both messages and system would inadvertently drop the system message due to override ordering.
Changes:
- Apply
prepareStep.messagesoverride before applyingprepareStep.system, ensuring the system message is preserved. - Add a changeset to publish the fix as a patch for
@workflow/ai.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| packages/ai/src/agent/stream-text-iterator.ts | Reorders prepareStep override application so system is applied after messages. |
| .changeset/fix-prepare-step-system-ordering.md | Adds a patch changeset entry documenting the bug fix. |
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| @@ -167,9 +172,6 @@ export async function* streamTextIterator({ | |||
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VaguelySerious
approved these changes
Mar 14, 2026
Apply messages override before system override so that the system message is prepended to the new prompt instead of being discarded. Previously, system was applied first then messages replaced the entire conversation prompt, losing the system message. Add tests for prepareStep system/messages ordering: - system only: prepended to prompt - system + messages: system prepended to replaced messages - system + messages with existing system: replaces existing - system updates across multi-step tool call rounds
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| '@workflow/ai': patch | ||
| --- | ||
| Fix `prepareStep` system message being discarded when `messages` is also returned |
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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 :)
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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?
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4 tasks
gr2m added a commit
to vercel/ai
that referenced
this pull request
Mar 23, 2026
## Summary Syncs DurableAgent code with changes from [vercel/workflow](https://github.com/vercel/workflow/tree/main/packages/ai/src/agent) since PR #12165 was opened (Jan 30, 2026). Key workflow PRs incorporated: - [#1362](vercel/workflow#1362) — AI SDK v6 migration, compat tests - [#1385](vercel/workflow#1385) — Various compatibility fixes - [#1389](vercel/workflow#1389) — Fix prepareStep system message lost when messages also returned - [#1329](vercel/workflow#1329) — Client-side tools support ## Changes - **Telemetry**: Add `recordSpan` wrapper around `model.doStream()` with OTel span attributes (provider, model, generation settings). New `telemetry.ts` module with lazy OTel API loading. - **`normalizeFinishReason`**: Handle both v5 string (`'stop'`) and v6 object (`{ unified: 'stop', raw: 'stop' }`) finish reason formats - **prepareStep ordering fix**: Apply `messages` override before `system` so the system message isn't lost when messages replaces the prompt - **Generic `DurableAgentOptions<TTools>`**: Tools and toolChoice properly typed through the generic parameter - **Constructor `prepareStep`**: New field on `DurableAgentOptions` for default step preparation across all `stream()` calls - **`TelemetrySettings`**: Add `recordInputs`/`recordOutputs` options - **Type helpers**: Export `InferDurableAgentTools` and `InferDurableAgentUIMessage` - **`sendStart` docs**: Explain when to set `false` (custom pre-stream UIMessageChunks) - **Telemetry threading**: Pass telemetry settings through to `executeTool` calls ## Test plan - [x] `pnpm type-check` — zero production code errors - [x] `pnpm check` — lint passes - [x] `pnpm build` in packages/durable-agent — builds - [x] `pnpm test:node` in packages/durable-agent — 60 passed, 22 expected fail (same as before) --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
gr2m added a commit
to vercel/ai
that referenced
this pull request
Mar 24, 2026
## Summary Syncs DurableAgent code with changes from [vercel/workflow](https://github.com/vercel/workflow/tree/main/packages/ai/src/agent) since PR #12165 was opened (Jan 30, 2026). Key workflow PRs incorporated: - [#1362](vercel/workflow#1362) — AI SDK v6 migration, compat tests - [#1385](vercel/workflow#1385) — Various compatibility fixes - [#1389](vercel/workflow#1389) — Fix prepareStep system message lost when messages also returned - [#1329](vercel/workflow#1329) — Client-side tools support ## Changes - **Telemetry**: Add `recordSpan` wrapper around `model.doStream()` with OTel span attributes (provider, model, generation settings). New `telemetry.ts` module with lazy OTel API loading. - **`normalizeFinishReason`**: Handle both v5 string (`'stop'`) and v6 object (`{ unified: 'stop', raw: 'stop' }`) finish reason formats - **prepareStep ordering fix**: Apply `messages` override before `system` so the system message isn't lost when messages replaces the prompt - **Generic `DurableAgentOptions<TTools>`**: Tools and toolChoice properly typed through the generic parameter - **Constructor `prepareStep`**: New field on `DurableAgentOptions` for default step preparation across all `stream()` calls - **`TelemetrySettings`**: Add `recordInputs`/`recordOutputs` options - **Type helpers**: Export `InferDurableAgentTools` and `InferDurableAgentUIMessage` - **`sendStart` docs**: Explain when to set `false` (custom pre-stream UIMessageChunks) - **Telemetry threading**: Pass telemetry settings through to `executeTool` calls ## Test plan - [x] `pnpm type-check` — zero production code errors - [x] `pnpm check` — lint passes - [x] `pnpm build` in packages/durable-agent — builds - [x] `pnpm test:node` in packages/durable-agent — 60 passed, 22 expected fail (same as before) --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
DurableAgent'sprepareStepwhere returning bothsystemandmessagescaused the system message to be discardedsystemoverride was applied first (prepended toconversationPrompt), thenmessagesreplacedconversationPromptentirely, losing the system messagemessagesfirst, thensystem, so the system message is prepended to the new promptDurableAgentusage whereprepareStepreturned a dynamic system prompt alongside modified messages (e.g. for context window management or message injection)