fix(streaming): support trailing-strings mode and error handling in non-beta messages

[Christian-Sidak]

Summary\n\nThe non-beta accumulate_event in _messages.py was missing two features that _beta_messages.py already had:\n\n- Support for partial_mode="trailing-strings" when the fine-grained-tool-streaming-2025-05-14 beta header is present. Without this, incomplete trailing strings in streaming tool inputs were silently dropped from intermediate snapshots rather than preserved as partial values.\n- A ValueError with a helpful message when the partial JSON is genuinely invalid, consistent with the beta path.\n\nThis PR adds request_headers to accumulate_event, threads the headers from both sync and async MessageStream.__stream__, and mirrors the conditional logic already in _beta_messages.py. This is a standard/beta streaming parity fix.\n\n## Test plan\n\n- [x] All existing streaming tests pass (tests/lib/streaming/)\n- [x] New TestAccumulateEvent tests cover: complete JSON with and without headers, trailing-strings mode activated by the beta header, and ValueError raised for invalid JSON in both modes

[00200200]

Thanks for bringing the standard messages path in line with the beta path. One scope issue before calling this Fixes #1925: the reported issue is a completed strict tool call where the outer arguments JSON is valid, but translations is a string containing nested JSON.

partial_mode="trailing-strings" only changes how incomplete JSON is represented while accumulating a stream. It cannot turn that completed string into an array or validate the tool schema, so get_final_message() would still expose ToolUseBlock.input["translations"] as a str for the payload in #1925. Nothing in this diff changes that behavior.

Could you either remove the Fixes #1925 link (and frame this as standard/beta streaming parity), or add a test that demonstrates the reported completed-response case is actually addressed? As written, the implementation looks useful, but the issue link suggests a fix for a server-side/schema-conformance problem it does not cover.

[00200200]

Found a blocking import mismatch in the new code and tests.

This repository imports the renamed client as httpx2, but the PR uses httpx.Headers at src/anthropic/lib/streaming/_messages.py:448 and throughout the new tests. Because of that:

• python -m pytest -q -n0 tests/lib/streaming/test_messages.py fails with 4 NameError: name 'httpx' is not defined failures.
• mypy reports src/anthropic/lib/streaming/_messages.py:448: error: Name "httpx" is not defined.

Please use httpx2.Headers consistently (or import the correct alias) before merging. The focused tests pass after that import mismatch is corrected; the current PR head is not green locally.

[Christian-Sidak]

Fixed in abe59d6:

• httpx2 alias: replaced httpx.Headers with httpx2.Headers throughout accumulate_event and the new tests to match the rest of the codebase.
• Fixes link: removed the Fixes #1925 link. The PR is framed as standard/beta streaming parity (bringing the non-beta accumulate_event in line with the beta path) rather than a fix for the server-side schema issue in Sonnet 5 strict tool use returns a string for an array-typed property #1925.