Anthropic streaming tool calls crash your JSON parser: input_json deltas are partial strings until content_block_stop

Stream JSON parse errors: accumulate, don't parse per delta

Symptom

JSON parse failures mid-stream, or tool calls with truncated arguments, only when using tool use with streaming.

Confirm the cause

input_json_delta events carry partial JSON strings, not complete objects. Parsing any single delta fails by design. Also check you are not choking on ping events or mid-stream error events, which the API can send at any time.

Fix

1. Accumulate partial_json strings per content block index; parse the concatenated string once, at content_block_stop.
2. Follow the event order: message_start, then per block content_block_start / deltas / content_block_stop, then message_delta, then message_stop.
3. Ignore ping events; handle error events as a failed attempt (see the 529 skill).
4. Prefer the SDKs: they accumulate and expose parsed incremental values for you.

Verify

A tool call with a large input object streams end to end and the parsed input at content_block_stop is a complete object matching the tool schema. Re-run with a slow network to confirm ping handling.

Partial JSON input deltas, the accumulate-and-parse-at-contentblockstop rule, ping events, and mid-stream error events are documented on the streaming page. Verified live at https://docs.anthropic.com/en/api/messages-streaming.