Problem

Multiple tool.execute.after hooks crash with TypeError: undefined is not an object (evaluating 'output.output.toLowerCase') when MCP tools return responses where output.output is undefined or not a string.

This affects v3.4.0 and was also present in v3.4.1 (which was yanked via #1720).

Error

TypeError: undefined is not an object (evaluating 'output.output.toLowerCase')

Reproduction

Any MCP tool that returns a response where output.output is undefined instead of a string will trigger this crash. This commonly happens with:

• MCP servers that return structured responses without a string output field
• Tools that return metadata-only responses
• Error conditions where the output field is not populated

Root Cause

Three tool.execute.after hooks access output.output without checking if it's a string first:

1. src/hooks/comment-checker/hook.ts (line 95):

   const outputLower = output.output.toLowerCase()  // CRASHES if undefined

2. src/hooks/edit-error-recovery/hook.ts (line 47):

   const outputLower = output.output.toLowerCase()  // CRASHES if undefined

3. src/hooks/task-resume-info/hook.ts (lines 24-25, 31):

   if (output.output.startsWith("Error:") || output.output.startsWith("Failed")) return  // CRASHES
   if (output.output.includes("\nto continue:")) return  // CRASHES
   output.output.trimEnd()  // CRASHES

Fix

Added if (typeof output.output !== 'string') return as an early guard in all three affected hooks, matching the existing pattern already used in src/hooks/tool-output-truncator.ts (line 42):

// tool-output-truncator.ts already does this correctly:
if (typeof output.output !== 'string') return

This is the minimal, consistent fix — it follows the established convention in the codebase and gracefully handles the case where output.output is undefined, null, or any non-string type.

Files Changed

Verification

• ✅ bun run typecheck — passes clean (0 errors)
• ✅ bun run build — passes clean
• ✅ All pre-existing tests unaffected

Related Issues

• FYI: v3.4.1 broke all external MCP tools (published & yanked) #1720 — v3.4.1 yanked due to this same TypeError class
• [Bug]: tool.execute.after hook crashes on /review command (output is undefined) #1035 — Same category of output.output crash (closed)

Summary by cubic

Fixes a TypeError crash in tool.execute.after hooks when MCP tools return non-string or undefined output.output. Adds a simple guard so structured or metadata-only responses no longer break execution.

• Bug Fixes
  • Add if (typeof output.output !== "string") return to:
    • src/hooks/comment-checker/hook.ts
    • src/hooks/edit-error-recovery/hook.ts
    • src/hooks/task-resume-info/hook.ts
  • Matches the existing pattern in tool-output-truncator and prevents crashes seen in v3.4.0/v3.4.1.

^{Written for commit 5a2075a. Summary will update on new commits.}