AnthropicContentBlock / antigravity ClaudeContentItem serialized `signature`
with omitempty, so thinking blocks without an upstream signature (Chat
Completions reasoning_content, Responses reasoning without encrypted_content,
every streamed thinking content_block_start) went out with no `signature` key.
Anthropic always sends the key (`""` on content_block_start). Strict clients
such as Grok Build deserialize it as a required field and abort the response
or SSE stream with "missing field `signature`".
- apicompat: thinking MarshalJSON always emits `thinking` and `signature`
- antigravity: ClaudeContentItem MarshalJSON does the same; streamed thinking
content_block_start carries `signature: ""`
- tests: wire-shape regression tests; update TestStreamingReasoning expectation
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
grok-build CLI recap-style side-calls (turn-summary / title-refresh) send
a fresh per-call X-Grok-Conv-Id label but set body prompt_cache_key to
the parent session id, so a direct xAI connection shares the main turn's
server-side prompt cache prefix.
sub2api's explicitGrokCacheSeed preferred X-Grok-Conv-Id over the body
key, which fragmented the cache identity on every side-call and forced a
full-price replay of the whole conversation (~300K+ tokens each, ~2.3%
cache hit rate observed over 44 side-calls / 4.9M input tokens in 7
days — about a third of all grok-4.6 input).
Priority is now: CC session > generic session headers > body
prompt_cache_key > X-Grok-Conv-Id > explicitKey > previous_response_id.
IDE header-stability tests updated; new regression test locks in the
side-call/parent key-sharing contract.
The responses->chat streaming converter re-emits the tool name as an empty
string on every function_call_arguments.delta because ChatFunctionCall.Name
has no omitempty. OpenAI-compatible clients accumulate the name from the
first tool-call delta, so the trailing "name":"" overwrites it and the
call fails dispatch (unknown tool ""). Empty names also break the next
round-trip upstream (missing field 'name').
Fixes#5631
