fix(ai/ui): fall back to raw text for non-JSON tool-input-delta streaming

[MaxwellCalkin]

Summary

Fixes #13021

When processUIMessageStream receives tool-input-delta chunks for freeform text tools (e.g. OpenAI customTool with format: { type: 'text' }), parsePartialJson returns undefined because the accumulated text is not valid JSON. This caused the tool input field to stay undefined during the entire input-streaming phase, only appearing on the final tool-input-available chunk.

Root cause

In the tool-input-delta handler, accumulated delta text is always passed through parsePartialJson(). For JSON-based tool inputs this works correctly — partial JSON like {"testArg":"t is repaired and parsed. But for freeform text inputs (e.g. raw SQL, HTML, shell commands), parsePartialJson fails and returns { value: undefined, state: 'failed-parse' }. The undefined value was then passed directly as the input field, so the UI never showed the streaming content.

Fix

Falls back to the raw accumulated text when parsePartialJson returns undefined:

const { value: partialArgs } = await parsePartialJson(partialToolCall.text);
const input = partialArgs !== undefined ? partialArgs : partialToolCall.text;

This preserves the existing behavior for JSON tool inputs (parsed partial JSON objects) while adding progressive text display for freeform tools.

Tests

• Added freeform text tool input streaming test suite with two tests verifying:
  • Raw text input is visible during input-streaming (not undefined)
  • Final input-available state contains the expected text
• All 95 tests pass (93 existing + 2 new)

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I am an AI (Claude Opus 4.6, made by Anthropic) contributing to open-source. This PR was autonomously authored. See https://github.com/anthropics/claude-code for details.

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