session: synthesize reasoning-start when stream omits it

[jackmazac]

Summary

Some model streams send reasoning updates without ever sending a matching “reasoning started” marker first. Downstream code then sees deltas or ends for an id it never opened, which can break ordering or UI assumptions. This adds a small stream normalizer so every reasoning id gets an explicit start before any delta or end.

Problem (plain language)

Think of reasoning like a paragraph: you expect “paragraph begins,” then sentences, then “paragraph ends.” If the provider skips the “begins” line and only sends middle sentences, anything that tracks open blocks by id gets confused. Users may see glitches or missing reasoning; worse, the stack may mishandle chunks.

Technical breakdown

1. The AI SDK exposes a wrapStream hook on language-model middleware.
2. Some providers emit reasoning-delta / reasoning-end for an id before a reasoning-start with that id.
3. We already normalize prompts in transformParams; this change normalizes the stream so each id is self-consistent.

Solution

• In packages/opencode/src/session/llm.ts, extend the existing stream middleware with wrapStream.
• Track ids that have received reasoning-start.
• On reasoning-delta or reasoning-end without a prior start for that id, enqueue a synthetic reasoning-start first, then the original chunk.

Files

• packages/opencode/src/session/llm.ts

How to test

From packages/opencode:

bun typecheck

Merge order

This PR does not depend on PR: session stripReasoning / compaction. It can land before or after that change. If both merge, behavior stacks: compaction strips reasoning from history, while this hook fixes live stream ordering for reasoning chunks.

[github-actions]

Hey! Your PR title session: synthesize reasoning-start when stream omits it doesn't follow conventional commit format.

Please update it to start with one of:

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[jackmazac]

Closing to reopen with a linked issue and the repo PR template per CONTRIBUTING.