can1357/oh-my-pi · error · ToolError

expression is required for evaluate

Error message

expression is required for evaluate

What it means

The evaluate action throws this ToolError when params.expression is missing. Evaluating an expression in the paused debuggee requires the expression string; there is no implicit default. Unlike breakpoint actions there is no capability gate — the check runs unconditionally.

Solutions

1. Provide the expression to evaluate, e.g. {expression: 'myVar.length', frame_id: 1}
2. Optionally set context ('repl', 'watch', 'hover', etc.) — it defaults to 'repl'
3. Guard empty interpolated expressions on the caller side before invoking the tool

Example fix

// before
await debugTool.run({ action: 'evaluate', expression: userInput }); // userInput was ''
// after
if (userInput.trim()) await debugTool.run({ action: 'evaluate', expression: userInput, frame_id: 1 });

if (typeof params.expression !== 'string' || params.expression.trim().length === 0) {
  throw new Error('evaluate needs a non-empty expression');
}

function hasExpression(p) {
  return typeof p === 'object' && p !== null
    && typeof (p as { expression?: unknown }).expression === 'string'
    && (p as { expression: string }).expression.trim().length > 0;
}

try {
  await debugTool.run({ action: 'evaluate', ...params });
} catch (err) {
  if (err instanceof ToolError && err.message.includes('expression is required')) {
    // skip or prompt for a real expression instead of evaluating an empty one
  } else throw err;
}

When it happens

Trigger: Calling action=evaluate with params.expression undefined, empty string, or whitespace-only (falsy).

Common situations: Agent sends an empty evaluate to 'refresh' watches; expression field named wrong (e.g. expr/code); template interpolation produced an empty string at runtime.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• data_id is required for remove_data_breakpoint
• data_id is required for set_data_breakpoint
• instruction_count is required for disassemble
• instruction_reference is required for…
• instruction_reference is required for…

AI-assisted analysis of can1357/oh-my-pi@9690622007 (2026-08-31). Data as JSON: /api/errors/e0611c6151934185. Report an issue: GitHub.

Appendix: source

Thrown at packages/coding-agent/src/tools/debug.ts:956

				details.state = outcome.state;
				details.timedOut = outcome.timedOut;
				return result.text(buildOutcomeText(outcome, timeoutSec, "Step in")).done();
			}
			case "step_out": {
				const outcome = await dapSessionManager.stepOut(combinedSignal, timeoutSec * 1000);
				details.snapshot = outcome.snapshot;
				details.state = outcome.state;
				details.timedOut = outcome.timedOut;
				return result.text(buildOutcomeText(outcome, timeoutSec, "Step out")).done();
			}
			case "pause": {
				const snapshot = await dapSessionManager.pause(combinedSignal, timeoutSec * 1000);
				details.snapshot = snapshot;
				return result.text(formatSessionSnapshot(snapshot).concat("Program paused.").join("\n")).done();
			}
			case "evaluate": {
				if (!params.expression) {
					throw new ToolError("expression is required for evaluate");
				}
				const evaluationContext = (params.context as DapEvaluateArguments["context"] | undefined) ?? "repl";
				const response = await dapSessionManager.evaluate(
					params.expression,
					evaluationContext,
					params.frame_id,
					combinedSignal,
					timeoutSec * 1000,
				);
				details.snapshot = response.snapshot;
				details.evaluation = response.evaluation;
				return result.text(formatEvaluation(response.evaluation)).done();
			}
			case "stack_trace": {
				const response = await dapSessionManager.stackTrace(params.levels, combinedSignal, timeoutSec * 1000);
				details.snapshot = response.snapshot;
				details.stackFrames = response.stackFrames;
				return result.text(formatStackFrames(response.stackFrames)).done();

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