fix(observer): emit <skip_summary/> on skip instead of empty response

[funrun11]

Problem

On quiet / low-signal sessions the SDK observer poison-loops and stores zero observations:

[PARSER]  SDK returned non-XML idle response — ignoring queued batch {consecutiveInvalidOutputs=1}
[PARSER]  SDK returned non-XML prose response — ignoring queued batch {consecutiveInvalidOutputs=2}
[PARSER]  SDK returned non-XML idle response — ignoring queued batch {consecutiveInvalidOutputs=3}
[ERROR]   SDK session poisoned — killing and respawning, pending messages preserved

Reproduced on 13.8.0 and 13.8.1 (Linux + Windows). Worker restart and version bump do not fix it.

Root cause — contract drift

src/sdk/prompts.ts instructs the observer:

Return either one or more <observation>...</observation> blocks, or an empty response if this tool use should be skipped.

and several mode skip_guidance fields repeat "return an empty response only" / "No output necessary if skipping."

But src/sdk/output-classifier.ts classifies an empty/non-XML reply as idle/prose → invalid output, and ResponseProcessor.ts poisons the session after 3 consecutive invalids. So a correct skip is scored as an invalid output and walks the session straight to the poison threshold.

Meanwhile the parser and the server-side providers already standardize on a self-closing <skip_summary/> tag:

• src/sdk/parser.ts — <skip_summary(?:\s+reason="([^"]*)")?\s*/>
• src/sdk/output-classifier.ts — treats <skip_summary/> as valid xml
• src/server/generation/providers/shared/prompt-builder.ts — "return a single self-closing <skip_summary />"

Fix

Align the SDK observation path and the mode skip_guidance with that existing contract: on skip, emit <skip_summary reason="..."/> (a recognized, valid output) instead of an empty response. No parser/threshold changes.

• src/sdk/prompts.ts — central skip instruction
• plugin/modes/code.json, meme-tokens.json, law-study.json, law-study--chill.json, email-investigation.json — skip_guidance

Testing

Patched a local install, restarted the worker, ran a quiet file-reading session. Before: poison loop, 0 observations. After: skips emit <skip_summary reason="..."/> (stored as valid, consecutiveInvalidOutputs stays 0), substantive reads distill into <observation> blocks and sync to chroma. No poison events.

Notes

Refs #3032, #3042. Complementary to the parser-side guards in #3059 / #2943 — this fixes the contract at the source (don't emit the empty reply) rather than tolerating it downstream. The two approaches compose.

[greptile-apps]

Greptile Summary

This PR aligns observer skip output with the existing XML skip contract. The main changes are:

• SDK observation prompts now request a single <skip_summary reason="brief reason" /> tag for skipped tool uses.
• Code, email investigation, law study, law study chill, and meme token modes now tell the observer not to return empty output on skips.
• The changed guidance matches the parser and output classifier paths that already treat <skip_summary /> as valid XML.

Confidence Score: 5/5

Safe to merge: the changes are limited to prompt guidance and mode configuration text that now matches the existing parser/classifier contract.

The modified files consistently replace empty skip-output instructions with the recognized skip tag, and no runtime parsing or threshold logic was changed.

T-Rex Logs

What T-Rex did

• T-Rex reviewed the pre-change prompts.ts state around line 148 describing empty response or no-output skip behavior.
• T-Rex confirmed the post-change state that prompts.ts now uses a single self-closing <skip_summary reason="brief reason" /> tag and that all changed modes have hasSkipSummaryBriefReason: true.
• T-Rex verified the parser/classifier results showing that a sample <skip_summary reason="brief reason" /> produced valid:true and skipped:true with classifyResult: xml, whereas an empty input produced valid:false and classifyResult: idle.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix(observer): emit <skip_summary/> on s..." | Re-trigger Greptile}