fix(observability): truncate raw SSE frames in error logs + better classification of mid-stream 400s

[kelsonpw]

Summary

Bug report: TUI Logs tab dumped raw Anthropic SSE protocol frames as [legacy] DEBUG text:

[2026-05-09T...] [legacy] DEBUG Agent result with error: API Error: 400 event: message_start
data: {"type":"message_start","message":{"model":"claude-sonnet-4-6",...}}
event: ping
data: {"type":"ping"}
event: content_block_start
... (hundreds more lines including partial_json tool_use deltas with file-path fragments)

Root cause — Part 1 (logging) — confirmed

src/lib/agent-interface.ts SDK Result-message handler at lines 4220 / 4249 calls logToFile('Agent result with error:', message.result) with NO truncation or SSE-frame stripping. When the Anthropic gateway terminates a stream with a 4xx, message.result contains the full failing SSE response body — kilobytes of event: / data: / partial_json lines. The own-comment at that callsite even says "Full message already logged above via JSON dump" — but that "JSON dump" was the unfiltered call.

Same leak in agent-runner.ts abortOnApiError / GATEWAY_DOWN / GATEWAY_INVALID_REQUEST / soft-error branches: rawMessage got interpolated straight into userMessage, WizardError ctx, and pushStatus — surfacing the SSE body verbatim in the TUI Outro screen and Sentry payloads.

Root cause — Part 2 (gateway 400 retry storm) — not addressed at source

The 400s themselves originate at the LLM gateway (intermittent — agent's tool_use payload was constructing a glob like ~/worktree-repos/Next* from partial_json fragments). The "6 errors of 6" the user saw is the wizard's existing inner-retry loop hitting 400s repeatedly. That's an upstream gateway issue and the existing classifier (classifyApiErrorSubtype) already routes it to terminated_400 with a clean user message. The fix here is purely "stop dumping the protocol noise" — the underlying retry-storm behavior is unchanged.

Fix

Add suppressSseFrames(msg) and sanitizeErrorMessageForLog(msg) to src/lib/agent-events.ts:

• Detect runs of SSE protocol frames (event: / data: / bare-JSON for the 8 known stream-event subtypes)
• Collapse each run into [N SSE frames suppressed] marker
• Cap result at existing 2KB MAX_LOG_MESSAGE_LENGTH
• Preserve non-frame content (real TypeError: ... / stack traces survive)

Apply at every leak site: both logToFile('Agent result with error:', ...) paths in agent-interface.ts, and the GATEWAY_DOWN / GATEWAY_INVALID_REQUEST / API_ERROR / RATE_LIMIT abort + soft-error branches in agent-runner.ts. Classification still runs on the raw form (the 400 terminated regex matches the head of the message, not the SSE body) — only logging / user-surface callsites use the sanitized form.

Verdict

Pre-existing leak, not a regression from main. The logToFile('Agent result with error:', message.result) callsite has been there since the original SDK Result-message handler. Unrelated to #698 — that PR only adds TUI per-event status tracking and does not touch error logging or the model-stream pipeline.

Test plan

• New unit tests src/lib/__tests__/agent-events-sse-suppression.test.ts (10 cases): fast-path no-op, contiguous-block collapse, inline-prefix split, real-error preservation alongside SSE noise, singular vs plural wording, bare-JSON form, unknown event-type passthrough, oversized-input pipeline (50KB SSE body → ≤2KB sanitized output)
• pnpm test — 254 files / 3815 tests pass
• pnpm test:bdd — 100 scenarios / 445 steps pass
• pnpm lint — clean (only pre-existing unrelated warning)
• pnpm build — clean

🤖 Generated with Claude Code

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Note

Low Risk
Low risk: changes are limited to sanitizing/truncating error strings before logging and displaying them, without altering agent control flow or API retry behavior.

Overview
Prevents raw Anthropic streaming (SSE) protocol frames from leaking into user-visible logs, status messages, and Sentry context by collapsing recognized stream frames into a single [.. SSE frame(s) suppressed] marker and then enforcing the existing MAX_LOG_MESSAGE_LENGTH cap.

Adds suppressSseFrames/sanitizeErrorMessageForLog in agent-events.ts, updates agent-interface.ts and agent-runner.ts to use the sanitized form anywhere SDK/gateway error strings are logged or interpolated (while keeping subtype classification on the raw message), and introduces focused unit tests covering frame detection, inline-prefix handling, preservation of real errors, and truncation behavior.

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