fix: accept empty string result in attempt_completion tool

[ritwikj2]

Summary

Fixes #11404 — The attempt_completion tool was incorrectly rejecting empty string results.

Problem

The existing code used a truthy check (if (!result)) which treated empty strings ('') as falsy, causing valid empty-string completions to be rejected with an error.

Solution

Changed the truthy check to an explicit existence check (if (result === undefined || result === null)) in NativeToolCallParser.ts, allowing empty strings to pass through as valid results.

Testing

Added 3 regression tests to verify:

• Empty string results are accepted
• Whitespace-only results are accepted
• Undefined/null results are still rejected

Checklist

• Code changes
• Tests added
• Type checks pass

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Reviewed commit: 9b3e7ba70a

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[roomote]

Reviewed the changes and verified locally. The fix is correct and well-tested.

Summary of review:

• The root cause is accurately identified: if (partialArgs.result) / if (args.result) treats empty string "" as falsy, causing valid empty-string completions to produce missing nativeArgs.
• The fix to !== undefined in both the streaming path (line 452) and the finalize path (line 781) is minimal and targeted.
• The three regression tests cover the key scenarios: empty string via parseToolCall, non-empty string via parseToolCall, and empty string via the streaming path (startStreamingToolCall + processStreamingChunk).
• All existing tests continue to pass alongside the new ones (15/15).
• Lint, prettier, and type checks all clean.

One small note: null values would also pass the !== undefined check (unlike the original truthy check which rejected both). This is fine in practice since result comes from JSON-parsed tool call arguments where null would be an unusual edge case, and the downstream handling of attempt_completion will still treat it appropriately. If extra defensiveness is desired, result != null (loose equality covering both null and undefined) could be used instead, but that is not a blocker.

LGTM.

[roomote]

Code Review Summary

The parser fix correctly allows empty strings through, but the downstream consumer still rejects them.

• AttemptCompletionTool.execute() in src/core/tools/AttemptCompletionTool.ts (line 72) still uses if (!result) which rejects empty strings as "missing param" -- this needs the same fix for the PR to actually resolve Tool execution fails: attempt_completion missing nativeArgs for Z-AI and Ollama after provider migrations #11404
• Use != null (loose inequality) instead of !== undefined in both parser locations to also reject null values while still allowing empty strings (lines 452 and 781)

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