Summary

• Add a centralized appendToOutput() helper to Prevent silent "undefinedstring" data corruption when MCP tools return undefined as output.output, complementing the TypeError crash fix in fix: guard output.output in tool after-hooks for MCP tools #1756 when MCP tools return undefined as output.output
• Complements fix: guard output.output in tool after-hooks for MCP tools #1756 (READ guards) by covering all remaining WRITE operations (output.output +=) across 13 hook files
• Includes 8 unit tests for the helper covering string, null, undefined, object, and sequential append scenarios

Changes

Problem

#1756 fixed READ operations (.toLowerCase()) on output.output with ?? "" guards in 3 files, but WRITE operations (output.output += text) remained unguarded across 13 hook files. When MCP tools (Atlassian, Exa, Chrome DevTools, grep.app, etc.) return undefined instead of a string for output.output, these writes cause:

• TypeError: undefined is not an object — crashes the entire tool execution pipeline
• "undefinedSome reminder text" — silent data corruption when JS coerces undefined + string

Solution

New helper (src/hooks/hook-output-guard.ts):

export function appendToOutput(output: { output: string }, text: string): void {
    if (output.output == null) {
        output.output = text   // initialize from null/undefined
        return
    }
    if (typeof output.output === "string") {
        output.output += text  // normal append
    }
    // non-string (object/array) → leave untouched to preserve structured responses
}

Design decisions:

• == null (loose equality) catches both null and undefined in one check
• Non-string values (objects, arrays) are left untouched to prevent corrupting structured MCP responses
• Initializes from null/undefined rather than silently skipping, so context injections are preserved

13 hook files updated — all output.output += ... and output.output = `${output.output}...` patterns replaced with appendToOutput() calls:

Testing

bun run typecheck   # passes
bun run build       # passes
bun test src/hooks/hook-output-guard.test.ts  # 8 pass, 0 fail

Test cases cover:

• Normal string append
• Empty string append
• undefined → initializes with text (core MCP bug scenario)
• null → initializes with text
• Non-string object → preserved (no corruption)
• Number → preserved
• Multiple sequential appends
• Multiple appends starting from undefined

Related Issues

Complements #1756 (d5fd918) — that PR guards READ operations, this PR guards WRITE operations.
Related to #1720 (original MCP tool output crash report).

Summary by cubic

Adds a centralized appendToOutput helper to make after-hook output writes null-safe, preventing crashes and "undefinedstring" corruption when MCP tools return undefined. Replaces ad-hoc string concatenation across hooks and adds tests.

• Bug Fixes
  • Introduced appendToOutput: initializes on null/undefined, appends to strings, leaves non-strings untouched.
  • Replaced output.output concatenations with the helper across 13 hooks; added a read guard in delegate-task-retry.
  • Added 8 unit tests; typecheck and build pass.

^{Written for commit 4b21310. Summary will update on new commits.}