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When an automatic compaction cut lands after the session's last user message (long agentic turns), the provider-bound history was [System(prompt), System(summary), Assistant, Tool, ...] with no user message. Z.ai/GLM rejects user-less message arrays with HTTP 400 "The messages parameter is illegal"; Anthropic and OpenAI tolerate them, which is why the failure was GLM-only and only right after a compaction. Map the persisted System-role summary through a new provider_bound_summary() role flip at the provider boundary so the outbound request is user-led. Persisted rows keep role System; the flipped copy is provider-bound only. The persisted SUMMARY_MESSAGE_PREAMBLE remains the single framing text. Tests: compaction.rs covers tool-only tail, mixed tail, and the empty tail (history is exactly [User(summary)] -- the minimal GLM-failing shape); engine.rs pins the full path through provider_history_messages_with_compaction + normalize_history_for_provider (made pub(crate) for the test) and asserts [System, User, Assistant, Tool] after the system prompt is prepended. All tests fail on revert.
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Problem
With GLM (Z.ai, OpenAI-compatible) models only, the first request after an automatic conversation compaction fails with HTTP 400
"The messages parameter is illegal". Anthropic and OpenAI models are unaffected. Notably, this even killed a long-running agent task mid-flight (the task's own session compacted and the next request 400'd).Root cause
When a compaction cut lands after the session's last user message (the norm in long agentic turns — user message, then a long assistant/tool chain), the provider-bound history is:
Z.ai rejects user-less message arrays; the other providers tolerate them — which is exactly why the bug was GLM-only and only right after a compaction.
Reproduced twice from this app's own history DB (runs
ee2ff7de,b4ca9e36in sessionb884a31a, after compactions1216b39eand5c1176ef): the message immediately after the compaction summary row had roleassistant, and the failing provider request carried no user message.Fix
At the provider boundary only, map the persisted System-role compaction summary through a new
provider_bound_summary()that emits role User:[System(prompt), User(summary), Assistant, Tool, ...]— user-led on every provider.normalize_history_for_provideralready merges consecutive user messages, so Anthropic never sees adjacent user turns; OpenAI passes roles 1:1; the CLI path filters summaries by metadata and never sees the flipped copy.SUMMARY_MESSAGE_PREAMBLEstays the single framing text — no double framing.Tests
All fail on revert (trace-verified):
compacted_history_with_only_tool_tail_leads_with_user_summary— user-less assistant/tool tail → history leads withUser(summary).compacted_history_with_empty_tail_is_summary_only— cut at the final session message → history is exactly[User(summary)], the minimal GLM-failing shape.compacted_history_with_mixed_tail_keeps_tail_after_user_summary— mixed tail ordering preserved.normalize_keeps_post_compaction_history_user_led(engine.rs) — goes through the realprovider_history_messages_with_compaction+normalize_history_for_providerwith a persisted-shape fixture, asserting[System, User, Assistant, Tool]after the system prompt is prepended.Validation
cargo check,cargo clippy --all-targets -- -D warnings,cargo fmt— all clean. (cargo testintentionally not run locally — it corrupts generated TS bindings; CI will run the suite.)Review
Three independent passes (two reviewer rounds + domain-expert final gate), verdicts recorded in the workspace: production_quality, no open findings.