[Bug] Anthropic SSE streaming silently drops malformed JSON frames

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Description

The Anthropic SSE parser silently ignores malformed JSON for message_start and content_block_delta events. A corrupted/incomplete event can therefore disappear from the stream without any error, leaving the consumer with a truncated response.

This is inconsistent with the OpenAI-compatible SSE parser, which explicitly throws on malformed JSON.

Location

src/providers/sse.ts:154-184 — parseAnthropicSseLine()

Relevant code

if (state.currentEvent === 'content_block_delta') {
  try {
    const parsed = JSON.parse(payload) as { delta?: { text?: string } };
    if (parsed.delta?.text) {
      return { kind: 'text', text: parsed.delta.text };
    }
  } catch {
    // Skip malformed JSON
  }
  return null;
}

The same silent catch exists for message_start.

By contrast, parseOpenAiSseLine() throws Malformed OpenAI SSE data: invalid JSON.

Steps to reproduce

1. Run commit-echo suggest --stream with the Anthropic provider.
2. Make the SSE stream contain a valid content_block_delta event followed by a malformed data: JSON payload and then message_stop.
3. The malformed event is ignored and the stream continues as if it were absent.
4. Depending on the remaining content, the command may end with truncated suggestions or a generic parse failure instead of identifying the malformed provider response.

This can be reproduced with a mocked fetch() response in the existing tests/stream-sse.test.mjs setup.

Expected behavior

Malformed provider frames should fail the stream with a clear error and release the response body.

Actual behavior

Malformed Anthropic frames in these event types are swallowed, so the caller cannot distinguish “provider sent bad JSON” from “provider sent no content.”

Suggested fix

Let JSON parse failures propagate with an Anthropic-specific error (matching the OpenAI behavior), and add regression tests for malformed message_start and content_block_delta frames.

Impact

Streaming responses can be silently truncated, producing misleading or unparseable commit suggestions and making provider-side protocol failures difficult to diagnose.

Reviewed against current main at c67ff967a018d5fd4b032f6ef6a4981ac9d76a12.

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Triage

Priority: P2 — streaming correctness

Silently dropping malformed Anthropic frames can produce partial or unparseable suggestions without identifying the protocol failure. That makes the failure mode substantially harder to diagnose than an explicit parser error.

Disposition: Fix alongside the streaming reliability work. Keep behavior consistent with the OpenAI-compatible parser and add malformed-frame regression coverage for both affected event types.

Suggested implementation: propagate JSON parse failures with an Anthropic-specific error while retaining the existing reader cleanup guarantees.

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Triage update

Disposition: P2 — valid bug; fix needed.

Confirmed against current main:

• parseAnthropicSseLine() still silently swallows JSON.parse() failures for both message_start and content_block_delta.
• The OpenAI-compatible counterpart was already fixed by [Bug] Malformed SSE JSON chunks are silently swallowed in streaming #269 / PR fix(streaming): report malformed SSE JSON #293, so this is a distinct Anthropic-side gap rather than a duplicate.
• streamSseResponse() already performs reader cleanup in its finally path, so propagating the parser error should preserve cleanup behavior.

Recommended implementation

• Propagate malformed JSON with an Anthropic-specific error such as Malformed Anthropic SSE data: invalid JSON.
• Add regression coverage for malformed message_start and content_block_delta frames.
• Include an end-to-end AnthropicProvider.completeStream() test with a valid chunk followed by malformed JSON to verify the error propagates and the response body is released.

Disposition: keep open and address as part of the streaming-reliability work.

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Implemented in PR #329: malformed Anthropic JSON in message_start and content_block_delta now throws Malformed Anthropic SSE data: invalid JSON instead of being silently discarded. Added parser regression tests for both event types and an end-to-end stream test that verifies the response body is released on the parser error. CI is running on the PR.