DeepSeek v4 (and other strict OpenAI-compatible providers) reject an
assistant message carrying ``tool_calls: []`` with HTTP 400 "Invalid
'messages[N].tool_calls': empty array. Expected an array with minimum
length 1, but got an empty array instead." Once it hits, every retry on
the session returns the same 400 and the conversation is stuck.
An empty array is semantically identical to "no tool calls", but it
reaches the wire from session resume, host-fed histories, and the
consecutive-assistant merge in repair_message_sequence (which preserves
a pre-existing []). None of the existing sanitizer passes touch it —
they all short-circuit on ``if not tcs`` / ``if msg.get("tool_calls")``.
Fix it at sanitize_api_messages, the final pre-API chokepoint, per the
#56980 review guidance: normalize on the per-call copy (shallow-copy the
message, drop the key) rather than in repair_message_sequence, which
would destructively rewrite the persisted trajectory and prompt cache.
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