fix: prevent CancellationError from killing ask_user tool loop

[sgaabdu4]

Problem

When ask_user is called by the LLM and then internally superseded (e.g., a new request arrives while a previous one is pending), the TaskSync extension was throwing vscode.CancellationError. This caused VS Code's Copilot Chat agent mode to permanently stop calling ask_user after just two cancellations.

Why this happens

VS Code's ToolCallingLoop treats CancellationError from a tool as a hard stop signal at three levels:

1. buildPrompt2() — checks ToolResultMetadata.isCancelled on all tool results. If any is cancelled, throws CancellationError
2. _runLoop() catch block — catches CancellationError and breaks out of the loop immediately
3. shouldAutoRetry() — excludes ChatFetchResponseType.Canceled from retry eligibility

The result: after two superseded ask_user calls, the conversation history contained two cancelled tool results, and the LLM learned that ask_user is broken and stopped calling it.

This is the same class of issue described in microsoft/vscode#241039 — "Cancelled tool call should not cancel the whole chat session, but rather respond to LLM with an error/special message for this specific tool call, and allow LLM to reason based on the received results."

Solution

1. Return normal results for superseded requests (tools.ts)

Instead of throwing CancellationError when a request is internally superseded, return the cancelled message as a normal AskUserToolResult. The LLM sees the text and can reason about it and call ask_user again.

Real user cancellation (Stop button) still correctly throws CancellationError via the CancellationToken / createCancellationPromise race path.

2. Instruct the LLM to re-ask the same question (toolCallHandler.ts)

Updated the superseded message from a generic "[CANCELLED: New request superseded this one]" to a directive that tells the LLM to re-ask the exact same question, reducing the chance of the question/summary changing on retry.

3. Tests updated (tools.test.ts)

• Updated 2 existing tests to assert normal result return instead of CancellationError throw
• Added new test verifying real token cancellation still throws CancellationError

Files Changed

File	Change
src/tools.ts	Return normal result for superseded requests instead of throwing CancellationError
src/webview/toolCallHandler.ts	Updated superseded message to instruct LLM to re-ask same question
src/tools.test.ts	Updated existing tests + added new test for real token cancellation

Validation

• Build: ✅
• TypeScript (tsc --noEmit): ✅ 0 errors
• Tests (vitest run): ✅ 437 tests passing
• Lint (biome check): ✅ 0 issues
• Code quality (check-code): ✅ All checks pass

References

• VS Code Language Model Tool API docs
• microsoft/vscode#241039 — Tool cancellation cancels whole chat (closed as duplicate, same root cause)
• ToolCallingLoop source — Where isCancelled check and loop break happen

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[sgaabdu4]

@n-r-w I created this PR as throw new vscode.CancellationError(); was killing ask_user tool in tools.ts from the changes yesterday. Would like your opinion as well @4regab

[sgaabdu4]

@4regab is 3 PRs good enough for a merge haha?

[Copilot]

Pull request overview

Prevents internally superseded ask_user calls from throwing vscode.CancellationError, which can terminate VS Code Copilot Chat’s tool-calling loop. This keeps the agent/tool loop resilient to overlapping ask_user requests while preserving real user-triggered cancellation behavior via the CancellationToken.

Changes:

• Return a normal AskUserToolResult (instead of throwing CancellationError) when a pending ask_user request is superseded.
• Update the superseded/cancelled message returned from the webview handler to instruct the LLM to re-ask the exact same question.
• Update/add Vitest coverage around superseded vs real token cancellation behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
tasksync-chat/src/tools.ts	Changes superseded handling to return a normal tool result instead of throwing CancellationError.
tasksync-chat/src/webview/toolCallHandler.ts	Updates the superseded/cancelled response text returned to the tool call.
tasksync-chat/src/tools.test.ts	Updates existing cancellation tests and adds a new token-cancellation test case.

[4regab]

@4regab is 3 PRs good enough for a merge haha?

Definitely! will review the PRs today

[4regab]

@n-r-w I created this PR as throw new vscode.CancellationError(); was killing ask_user tool in tools.ts from the changes yesterday. Would like your opinion as well @4regab

I haven’t personally encountered this yet, so I’ll need to test it more thoroughly later.