fix(ai): keep Anthropic stream tool arguments after server tool blocks

[breken-ai]

💡 Motivation and Context

Anthropic stream events address content blocks by their index in the message. _AnthropicStreamAccumulator passed content_blocks to handle_anthropic_tool_delta and finalize_anthropic_tool_input, which look the block up with content_blocks[event.index]. But content_blocks only holds the block types the wrapper formats (text, thinking, redacted_thinking, tool_use). A server_tool_use or web_search_tool_result block still takes an index in the stream and is skipped in the list.

So once a response uses a server tool (web search, web fetch, code execution) before a client tool call, the list and the stream index no longer line up:

• the tool call's input_json_delta events point past the end of the list, and $ai_output_choices records the call with arguments: {};
• if a tool call came before the server tool, the server tool's input deltas land on the next tool block and its own arguments are replaced.

The fix keeps a second list that lines up with the stream index (an empty placeholder for block types that are not formatted) and uses it only for tool deltas and finalization. content_blocks and the captured output are unchanged.

💚 How did you test it?

• New test test_streaming_tool_call_after_server_tool_blocks builds the stream from the real anthropic.types events: server_tool_use (0), web_search_tool_result (1), text (2), tool_use with two input_json_delta chunks (3). It goes through the wrapped Anthropic client.
  • On main (4a138e6) it fails: the tool call is captured with 'arguments': {}.
  • With the fix it passes with {'city': 'SF'}.
• pytest posthog 4144 passed, 16 skipped. ruff format --check, ruff check, mypy through mypy-baseline filter and python -W error -c "import posthog" are clean.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

• Found and fixed by an autonomous agent (Claude Code, Claude Opus) while reviewing the AI wrappers' streaming accumulators. The failing test was run against main before the fix.
• I kept the fix inside the accumulator instead of changing the handle_anthropic_* helper signatures, so the non-streaming path and the formatted output stay as they are.

Commits here are unsigned; this repo squash-merges, so GitHub signs the merged commit.

[dustinbyrne]

thanks @breken-ai

this looks correct to me (and if it is, we could benefit from the same strategy in posthog-js) - worth @PostHog/team-ai-observability signing off

[breken-ai]

@dustinbyrne thanks for the merge. I checked posthog-js for the same bug on main (c95ce40). It's already fixed there, so there's nothing to port.

packages/ai/src/anthropic/index.ts (PostHogAnthropic streaming): fixed in PostHog/posthog-js#5113 (merged 2026-09-30). toolsInProgress is now a Map keyed by the stream's chunk.index. The input_json_delta and content_block_stop handlers look up toolsInProgress.get(chunk.index) and write to the block object that content_block_start pushed into contentBlocks. Position in contentBlocks no longer matters. server_tool_use, web_search_tool_result and thinking blocks never enter the map, so their deltas miss it and are skipped. The JS fix differs from the one here (a map instead of an aligned list), but it covers the same case.

The test #5113 added covers the thinking-block case. To check the server-tool cases, I ran a local test (not committed) through the wrapped client with the same block layouts as test_streaming_tool_call_after_server_tool_blocks:

• server_tool_use (with its own input_json_delta), web_search_tool_result, text, then tool_use: arguments: {"city": "SF"} on main. On #5113's parent (41f12438^) it is arguments: {}.
• tool_use first, then the server tool blocks: {"city": "SF"} on both. The server tool's deltas don't overwrite the client tool's arguments.

packages/ai/src/claude-agent-sdk/processor.ts (processStreamEvent): this is the other Anthropic stream consumer. It stores streamedOutput[event.index], a sparse array keyed by the stream index, and deltas read streamedOutput[event.index]. So the lookup always lines up. formatAssistantBlocks maps every non-null block to exactly one item (server tool blocks become a JSON-text item), so the slice(pendingOutput.length) in _captureGeneration also lines up with stream indexes. I checked this with a local interrupted stream: server_tool_use, then web_search_tool_result, then a partial tool_use. The tool keeps '{"file_path":"/tmp'.

No other file in packages/ai/src reads content_block_* or input_json_delta events. The OpenAI accumulator keys tool calls by toolCall.index in a Map, and the Vercel, LangChain and ADK paths don't use Anthropic block indexes.