[Bug] Claude API Error: Malformed tool-invocation tag with leaked control tokens after context compaction

Bug Description
### Title Model intermittently emits literal "court" + malformed tool-call tag instead of a valid tool invocation (calls are dropped) ### Environment - Claude Code: 2.1.186 - OS: macOS (Darwin 25.5.0) - Model: claude-opus-4-8 (Opus 4.8); also reproduced after switching to Opus 4.8 1M context - Session id: f0861d81-280a-48ce-8651-4cacd7cf2e68 ### Summary During an agentic session, the assistant intermittently emits a stray literal text token court immediately followed by a malformed tool-invocation tag (e.g. <invoke name="Bash"> lacking the correct antml: namespace / function_calls wrapper) instead of a well-formed tool-call block. The harness then rejects it with: "Your tool call was malformed and could not be parsed." ### Observed behavior - The intended tool call (Bash, etc.) is silently dropped — the step does not run. - The stray court text is visible in the assistant's output, looking like leaked control tokens. - It recurs even immediately after the assistant correctly explains the issue and explicitly tries to avoid it, i.e. the model cannot self-correct within the session. ### Conditions / frequency - Frequency increased markedly AFTER a /compact (context compaction) and in a long-running session. - Persisted across a model switch (to Opus 4.8 1M context). - When the same call is emitted correctly, it executes normally (so the command itself is fine; only the invocation markup is corrupted). ### Impact - Blocks tool use; the user must repeatedly wait for retries or run commands manually via the ! prefix. - No data loss, but it is confusing and resembles prompt injection (it is not — court does not originate from any user input, repo file, or tool output). ### Expected behavior The model should always emit a well-formed tool-invocation block, or the harness should recover gracefully instead of dropping the call. ### Repro notes Not deterministically reproducible, but reliably recurring in this session after /compact. Full transcript available via the session id above.

Environment Info

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.186
• Feedback ID: 65483a14-bc82-4b90-9a4a-b66447ca8e07

Errors

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