fix: stop attaching accumulated reasoning_details to reasoning-delta events (#413)

[robert-j-y]

Summary

Fixes #413 — reasoning metadata O(N²) payload bloat.

Problem

Each reasoning-delta SSE event carried a full snapshot of all accumulatedReasoningDetails in its providerMetadata. For N reasoning chunks, total serialized payload grew O(N²) instead of O(N).

For example, with 20 reasoning chunks the total payload was 10,537 bytes — well above the 6,000-byte linear threshold (300 bytes × 20 chunks).

Fix

reasoning-start and reasoning-delta events no longer carry providerMetadata. The full accumulated reasoning_details are still available on reasoning-end, tool-call, and finish events (unchanged).

Before / After

Before: Every reasoning-delta included a growing snapshot:

reasoning-delta { delta: "chunk 1", providerMetadata: { reasoning_details: [detail1] } }
reasoning-delta { delta: "chunk 2", providerMetadata: { reasoning_details: [detail1, detail2] } }
reasoning-delta { delta: "chunk 3", providerMetadata: { reasoning_details: [detail1, detail2, detail3] } }
// ... O(N²) total payload

After: Deltas carry only the text:

reasoning-delta { delta: "chunk 1" }
reasoning-delta { delta: "chunk 2" }
reasoning-delta { delta: "chunk 3" }
// reasoning_details still available on reasoning-end and finish events

Changes

• src/chat/index.ts — Removed providerMetadata parameter from emitReasoningChunk, removed snapshot creation
• src/chat/index.test.ts — Updated 3 test expectations to verify providerMetadata is undefined on reasoning-delta/start events
• e2e/issues/issue-413-reasoning-metadata-bloat.test.ts — 7 new regression tests
• .changeset/fix-413-reasoning-metadata-bloat.md — Patch changeset

Test Results

• Unit tests: 400 passed
• Regression tests (issue-413): 7/7 passed
• pnpm stylecheck: passed
• pnpm typecheck: passed

Human Review Checklist

• Verify that consumers relying on providerMetadata from reasoning-delta events are not broken (this was undocumented behavior)
• Confirm reasoning-end and finish events still carry the full reasoning_details
• Check that multi-turn reasoning with tool calls still works correctly

---

Generated by Devin