fix(ai): avoid crashing on malformed streamed tool-call input

[craze3]

Description

Closes #1706

• add a small safe parser for streamed tool-call.input
• replace eager JSON.parse(...) calls in do-stream-step.ts and stream-text-iterator.ts that could crash the workflow step before experimental_repairToolCall was reached
• preserve malformed input as the raw string instead of throwing so existing repair and error-handling paths remain reachable

This keeps the fix narrowly scoped to internal DurableAgent reconstruction paths. It does not change the public API or introduce provider-specific behavior.

How did you test your changes?

• added unit coverage for the new safe parser in packages/ai/src/agent/do-stream-step.test.ts
• added a regression test that exercises doStreamStep with a mock streamed tool call whose input is malformed JSON and verifies the step no longer throws
• added a regression test in packages/ai/src/agent/stream-text-iterator.test.ts that verifies malformed tool-call input is preserved into the reconstructed assistant message instead of crashing during prompt rebuild
• ran the focused @workflow/ai test suite for the touched agent paths:

pnpm --filter @workflow/ai test -- \
  src/agent/do-stream-step.test.ts \
  src/agent/stream-text-iterator.test.ts \
  src/agent/durable-agent.test.ts \
  src/agent/telemetry.test.ts

• ran the package build:

pnpm --filter @workflow/ai build

Recommended commands to rerun in a fresh fork before marking ready:

pnpm --filter @workflow/ai test -- \
  src/agent/do-stream-step.test.ts \
  src/agent/stream-text-iterator.test.ts \
  src/agent/durable-agent.test.ts \
  src/agent/telemetry.test.ts
pnpm --filter @workflow/ai build

Notes

• This intentionally preserves the raw malformed input string on parse failure rather than coercing to {} so repair hooks and debugging still have access to the original provider output.
• This does not attempt to repair malformed input itself; it only prevents internal reconstruction from crashing before repair/error handling can occur.

PR Checklist - Required to merge

• 📦 pnpm changeset was run to create a changelog for this PR
• 🔒 DCO sign-off passes
• 📝 Ping @vercel/workflow in a comment once the PR is ready, and the above checklist is complete

[changeset-bot]

🦋 Changeset detected

Latest commit: 54db77b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@workflow/ai	Patch

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[vercel]

@craze3 is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[craze3]

@vercel/workflow ready for review.

This fixes #1706 by preserving malformed streamed tool-call input during DurableAgent reconstruction so experimental_repairToolCall and normal error handling can run instead of crashing early.

It looks like the Vercel preview checks on this fork PR are blocked pending team authorization.

[VaguelySerious]

LGTM generally, I was concerned about leaving the malformed string in there potentially, so I had AI do a quick assessment. Let me know what you think about it / adjust as you see fit, then feel free to re-tag me for review here or send a comment on the issue

---

If the repair function fixes the tool call and execution succeeds, the loop continues and the prompt is sent to the provider on the next turn. Providers serialize this input directly:

• OpenAI (@ai-sdk/openai, internal/index.mjs:224): does arguments: JSON.stringify(part.input). If input is already a string like '{"city":"San', JSON.stringify produces a double-encoded
  string literal '"{\"city\":\"San"' — not a valid function arguments value.
• Anthropic (@ai-sdk/anthropic, internal/index.mjs:2451): passes input: part.input directly as tool_use.input. Anthropic's API expects a JSON object here; a string will be rejected.

So the flow is:

1. Model emits malformed tool-call input
2. safeParseToolCallInput preserves it as a string (no crash — good)
3. String is baked into conversationPrompt
4. executeTool in durable-agent.ts:1581 hits JSON.parse → throws → repair runs → tool executes successfully
5. Next doStreamStep call sends the contaminated prompt to the provider
6. Provider API error on the next turn — a new failure that didn't exist before

Before this PR, the crash at step 2 prevented the loop from ever reaching step 5-6. After the PR, you trade an immediate crash for a deferred one on the next model call. This only
matters when all three conditions hold: malformed input + repair succeeds + there's a subsequent model turn. But that's exactly the scenario the PR is designed to enable.

Suggested fix:
After repair succeeds in executeTool, the repaired tool call input should be patched back into the conversationPrompt. Alternatively, streamTextIterator could defer writing the
assistant tool-call message until after tool execution/repair, using the (potentially repaired) input.

[VaguelySerious]

I think this can just be original.safeParseToolCallInput instead of rewriting the function, to avoid drift

[craze3]

Moved the parser into a shared internal helper, so stream-text-iterator now imports the real implementation instead of re-declaring it in the test mock.

[VaguelySerious]

Could this also re-use safeParseInput from durable-agent.ts? Or is the undefined fallthrough intentional?

[craze3]

Pulled this into a shared helper and now reuse it from do-stream-step, stream-text-iterator, and durable-agent. The undefined/empty-input -> {} behavior is still intentional for no-args tool calls.

[craze3]

Good catch on the deferred-failure path. I pushed a follow-up commit that patches repaired tool-call input back into the accumulated assistant tool-call message before the next model step, and also updates the original tool-call object so downstream toolCalls / toolResults reflect the repaired value too.

I also moved the parser into a shared helper to avoid drift between the touched files and tests. Re-ran the focused @workflow/ai checks after the change:

pnpm --filter @workflow/ai test -- \
  src/agent/do-stream-step.test.ts \
  src/agent/stream-text-iterator.test.ts \
  src/agent/durable-agent.test.ts \
  src/agent/telemetry.test.ts
pnpm --filter @workflow/ai build