Write a client-side SSE parser for streamed Claude Messages responses that reassembles text, tool_use input JSON, and thinking blocks

Documented steps

1. Set stream:true on the /v1/messages request and consume the SSE stream.
2. Expect this event order: message_start (a Message with empty content), then per content block content_block_start (carries index), one or more content_block_delta, and content_block_stop; then one or more message_delta; then message_stop.
3. Treat ping events as no-ops that may appear anywhere.
4. Text blocks: accumulate content_block_delta events with delta.type == 'text_delta', concatenating delta.text per block index.
5. tool_use blocks: accumulate delta.type == 'input_json_delta' fragments from delta.partial_json per index, and only JSON-parse the accumulated string after content_block_stop for that index. The final tool_use.input is always an object.
6. thinking blocks: accumulate delta.type == 'thinking_delta' from delta.thinking; the signature arrives separately as delta.type == 'signature_delta' with delta.signature just before content_block_stop.
7. Enable eager (fine-grained) tool streaming per tool with eager_input_streaming:true in the tool definition. The legacy header anthropic-beta: fine-grained-tool-streaming-2025-05-14 enables it for tools that do not set the field; an explicit eager_input_streaming:false keeps buffered streaming even with the header present.
8. Because eager streaming skips server-side JSON validation, wrap the accumulated parse in try/except. On failure — including truncation when max_tokens is hit mid-parameter — return a tool_result with is_error:true containing the raw unparseable string.
9. Handle out-of-band error frames anywhere in the stream, e.g. {"type":"error","error":{"type":"overloaded_error"}}, which corresponds to HTTP 529.
10. Prefer the official SDK streaming helpers, which already do partial-JSON accumulation and expose assembled values.
11. Official docs: https://platform.claude.com/docs/en/build-with-claude/streaming