[Bug] Tool calls intermittently emitted as plain text instead of structured blocks, causing parse failures and hard hangs on retry

Bug Description
Title: Tool calls intermittently emitted as plain text ("could not be parsed"), self-perpetuating on retry → hard hang Environment: - Claude Code 2.1.161 (latest), native install, WSL2 (Ubuntu 24.04) on Windows 11 - Model: Opus 4.8 (claude-opus-4-8, 1M context) - Heavy Japanese-language sessions; many concurrent background sessions; 7 MCP servers Description: Intermittently the model emits tool-call markup as literal TEXT instead of a structured tool_use block. The harness returns: "Your tool call was malformed and could not be parsed. Please retry." The retry frequently reproduces the same corruption, yielding: "The model's tool call could not be parsed (retry also failed)." which hard-stops the turn until the user manually sends another message. Signature (from session JSONL): A spurious token (observed literally as court) appears where the tool-call opening delimiter should be, followed by the <invoke> markup rendered as text. Minimal real example — an 84-character assistant message that was ENTIRELY: court <invoke name="Bash"><parameter name="command">echo test</parameter></invoke> This is a trivial call, so the failure is NOT correlated with payload size/content. Frequency: In one environment over 12 days: 87 first-attempt failures + 46 "retry also failed" fatal stalls (~4 fatal/day). Reproduced live many times while debugging. Impact: Sessions appear frozen "at tool execution" for tens of minutes — worst in background/parallel sessions that aren't actively watched, because the fatal variant needs a manual nudge to recover. Additional notes: - Affects Bash/Write/Read/Edit equally; trivial calls fail too. - Appears more often in Japanese-context turns and under high tool-call volume. - autoUpdatesChannel=latest; confirmed present on the latest release (2.1.161).

Environment Info

• Platform: linux
• Terminal: wsl-Ubuntu-24.04
• Version: 2.1.161
• Feedback ID: 60f2e4c3-bc8b-4dcb-b55c-9ab2e4dc2098

Errors

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