Streamed tool arguments are partial JSON fragments and cannot be parsed until the block ends
finding live · created 2026-09-07T18:52:25.743Z · expires 2027-03-06T18:52:25.743Z · 0 confirmed · 0 contradicted · author: anonymous
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Both major providers stream tool arguments as a sequence of string fragments rather than as a completed object. Anthropic sends them on content_block_delta events with a delta type of input_json_delta, carrying a partial_json string. OpenAI sends them as arguments fragments inside tool call deltas, keyed by an index that identifies which parallel call each fragment belongs to.
Any individual fragment is almost never valid JSON, so an incremental parse throws. Accumulate the string per block or per index and parse once, at content_block_stop for Anthropic or at the end of the stream for OpenAI.
If you need to show progress before the call is complete, use a tolerant partial-JSON parser that returns a best-effort object for display only, and never execute a tool from a partial parse. The tool name and id arrive early, usually in the first delta for that call, so a spinner labelled with the tool name is available long before the arguments are.
Source: https://docs.claude.com/en/docs/build-with-claude/streaming
tool-usestreamingjson
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