fix(agent): stream tool input as it arrives during execution

[skoob13]

Problem

When a tool is running, the conversation row shows the tool name but no input args. The args only appear once the tool result lands. For fast tools the gap is invisible; for slow ones (PostHog MCP HTTP calls especially) the user stares at an in-flight row with empty args.

Cause: the agent emits the tool_call from the Anthropic content_block_start event, which always carries input: {}. The full input streams in via input_json_delta events, which the chunk handler was dropping. The full rawInput only ever reached the renderer when the finalized SDKAssistantMessage arrived — sometimes after the tool had already returned.

Changes

• New tryParsePartialJson helper that walks the streamed JSON tracking quote/escape/bracket state and tries a couple of completions (close any open string, drop trailing comma/colon/partial key, balance brackets). Returns null if no completion parses, so callers silently skip un-parseable deltas.
• streamEventToAcpNotifications now:
  • On content_block_start for tool_use / mcp_tool_use, registers a per-content-block-index buffer.
  • On content_block_delta with input_json_delta, appends the partial JSON, parses it best-effort, and emits a tool_call_update carrying the parsed-so-far rawInput.
  • On content_block_stop, clears the per-index entry.
• The cache is also clear()ed in prompt()'s finally so a cancelled or errored turn can't leak partial entries into the next turn.

How did you test this code?

• 11 unit tests in partial-json.test.ts covering empty input, complete JSON, mid-stream state (open string, missing closer), escaped quotes, partial keys, nested objects/arrays, an incremental simulation of a real exec command, and the unparseable-garbage path.
• pnpm --filter agent typecheck clean; full agent suite (313 tests) passes.
• Manually exercised in PostHog Code against a slow mcp__posthog__exec call execute-sql {...} invocation: the args now stream into the row in real time instead of appearing only after the tool returned.

Publish to changelog?

no

[greptile-apps]

Prompt To Fix All With AI

Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
packages/agent/src/adapters/claude/types.ts:79-85
**`toolName` is stored but never read**

`toolName` is written into every cache entry but is not accessed in `inputJsonDeltaToAcpNotifications` (or anywhere else in the diff). This is a superfluous part — only `toolUseId` and `partialJson` are actually used. Removing it keeps the type honest and avoids confusion about why the name was cached.

### Issue 2 of 3
packages/agent/src/utils/partial-json.test.ts:5-14
**Prefer parameterised tests**

Several `it` blocks group multiple independent input/output pairs as sequential `expect` calls. The team prefers `it.each` for this pattern so each case gets its own label and failure message. For example:

```ts
it.each([
  ["", null],
  ["   ", null],
] as const)("returns null for %j", (input, expected) => {
  expect(tryParsePartialJson(input)).toBe(expected);
});
```

This pattern also applies to "drops a trailing partial key" and "handles nested objects and arrays mid-stream". The incremental-sequence test is fine as-is since it tests a stateful progression rather than independent cases.

### Issue 3 of 3
packages/agent/src/utils/partial-json.ts:13
**`unknown | null` return type is redundant**

`null` is already assignable to `unknown`, so `unknown | null` simplifies to `unknown`. The explicit `| null` adds noise without adding type safety — callers still have to narrow via `!parsed` check anyway.

_{Reviews (1): Last reviewed commit: "fix(agent): stream tool input as it arri..." | Re-trigger Greptile}