fix(server): key assistant text blocks by message id instead of stream position

[Kiri110K]

The deeper of two alternative fixes for #7137 — the small one is #7142. Pick one, they are not meant to be merged together. This one changes how assistant text blocks are identified; #7142 keeps positional matching and just repairs its alignment.

What changed

ClaudeAdapter tracked assistant text blocks in one turn-long list and matched snapshot text to blocks by list position. A claude/assistant snapshot describes one message, not the whole turn, so any turn with more than one assistant message could mismatch.

The adapter now reads message_start — previously ignored entirely — and keys every text block by its message id plus the block index within that message. Snapshot backfill resolves blocks of its own message only. A block whose stream was cut mid-message is completed from the snapshot when the snapshot text extends the streamed prefix; a snapshot that contradicts already-shown text is never written over it (warning logged instead).

Why this fixes four defects with one root cause

• A stalled stream truncates the message to its first delta — the incident in [Bug]: A stalled text stream silently truncates the assistant message to its first delta #7137: one text_delta (P), 3m16s of silence, no content_block_stop, full 2.4 KB snapshot at the end. Positional matching wrote the snapshot onto an earlier completed block and the real block was force-completed empty.
• Multi-text-block messages (text → tool call → text) could backfill the wrong block, since position-in-turn and position-in-message disagree.
• Two consecutive messages merged into one when the second message restarted block indexes at zero while the first was still open.
• A subagent's content_block_stop closed the parent's open block, truncating the parent message mid-stream.

Worth noting for review: open PR #6429 copies the positional scheme into new backfillReasoningBlocksFromSnapshot for thinking blocks. If it lands on top of this PR, reasoning gets the message-scoped base instead of inheriting the same truncation bug.

Tests

• Four new regression tests, one per defect above, each replaying the SDK event sequence; all four fail without the fix and pass with it.
• Full server suite: 230 files passed, 2510 tests passed, 7 skipped, on top of current main.

Diff is +147/−40 source; the rest is tests.

🤖 Generated with Claude Code

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Note

Medium Risk
Changes core Claude streaming-to-UI mapping for assistant messages; wrong block association would still truncate or duplicate chat text, though behavior is heavily regression-tested.

Overview
ClaudeAdapter now scopes assistant text blocks by API message.id (from message_start) plus content-block index, instead of matching snapshot text to blocks by position in a turn-wide list.

When a stream stalls and the CLI delivers a full claude/assistant snapshot, backfill targets the correct message’s block and emits only the unseen text suffix as content.delta (tracked via streamedText), avoiding truncated or duplicated assistant messages. Snapshot matching no longer completes blocks on snapshot alone unless the stream already closed; divergent snapshot text is skipped with a warning.

content_block_stop from subagent traffic no longer closes the parent’s open text block. Synthetic snapshot-only messages use content index + message id instead of negative synthetic block indices.

Five new regression tests replay stalled streams, multi-block repair, reused block indexes across messages, subagent stops, and snapshot-only second messages.

^{Reviewed by Cursor Bugbot for commit d14de2d. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix assistant text blocks to key by message ID instead of stream position in ClaudeAdapter

• Assistant text blocks in ClaudeAdapter.ts are now keyed by both block index and API message ID, preventing content from different API messages being merged into the same chat item.
• Snapshot-based stream repair now emits only the unseen suffix of fallback text, preventing duplicate text from appearing on clients when a stalled stream is repaired.
• Subagent content_block_stop events no longer truncate the parent assistant message; subagent traffic is filtered by checking for a parent_tool_use_id.
• message_start events capture the current assistant message ID into turn state so subsequent content blocks are correctly associated with their parent message.
• Five new integration tests cover streaming repair, multi-block messages, index reuse across messages, and subagent event isolation.

^{Macroscope summarized d14de2d.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This bug fix modifies core streaming state machine logic with non-trivial changes to how text blocks are associated with messages during real-time streaming. The complex matching logic and multiple fallback paths warrant human review despite thorough test coverage.

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