fix: add null guards for output.output in tool.execute.after hooks

[code-yeongyu]

Fixes #1746

Problem

Three tool.execute.after hooks crash with TypeError: undefined is not an object (evaluating 'output.output.toLowerCase') when MCP tools return results where output.output is undefined. This breaks ALL external MCP tools since v3.5.0.

Affected Hooks

File	Line	Crash
comment-checker/hook.ts	L95	output.output.toLowerCase()
edit-error-recovery/hook.ts	L47	output.output.toLowerCase()
task-resume-info/hook.ts	L24-31	.startsWith(), .includes(), .trimEnd()

Fix

Added if (!output.output || typeof output.output !== "string") return guard at the top of each affected hook handler.

Testing (TDD)

• Tests for each hook with undefined output.output — all pass
• Existing tests unaffected

Related

• TypeError: output.output.toLowerCase() crashes on MCP tools with undefined output #1737, FYI: v3.4.1 broke all external MCP tools (published & yanked) #1720 (user reports)
• PR fix: guard against undefined output.output in tool.execute.after hooks #1735 (community fix attempt)
• MCP tools crash: TypeError on value.output.output in processor.ts (AI SDK 6.0.74 breaking change) anomalyco/opencode#13042 (upstream AI SDK 6.0.74 breaking change)

---

Summary by cubic

Prevent crashes in tool.execute.after hooks caused by undefined output from MCP tools, restoring compatibility with external tools. Fixes #1746.

• Bug Fixes
  • Added string guards and early returns for output.output in comment-checker, edit-error-recovery, and task-resume-info.
  • Added tests to cover undefined output in all three hooks.

^{Written for commit 8b44d3b. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 7 files

Confidence score: 4/5

• The main concern is in src/hooks/comment-checker/hook.ts: an early return on missing output.output can skip comment checks for write/edit/apply_patch cases, which could let unchecked changes through.
• This is a moderate-risk behavioral gap rather than a crash, so the PR still looks safe to merge with minor caution.
• Pay close attention to src/hooks/comment-checker/hook.ts - ensure missing output is handled without skipping comment checks.

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="src/hooks/comment-checker/hook.ts">

<violation number="1" location="src/hooks/comment-checker/hook.ts:92">
P2: Early-returning on missing output.output prevents the hook from running comment checks for write/edit/apply_patch cases where tools omit output.output. Instead of returning, coerce to an empty string so the hook can still process metadata/pending calls.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P2: Early-returning on missing output.output prevents the hook from running comment checks for write/edit/apply_patch cases where tools omit output.output. Instead of returning, coerce to an empty string so the hook can still process metadata/pending calls.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/hooks/comment-checker/hook.ts, line 92:

<comment>Early-returning on missing output.output prevents the hook from running comment checks for write/edit/apply_patch cases where tools omit output.output. Instead of returning, coerce to an empty string so the hook can still process metadata/pending calls.</comment>

<file context>
@@ -89,6 +89,8 @@ export function createCommentCheckerHooks(config?: CommentCheckerConfig) {
     ): Promise<void> => {
       debugLog("tool.execute.after:", { tool: input.tool, callID: input.callID })
 
+      if (!output.output || typeof output.output !== "string") return
+
       const toolLower = input.tool.toLowerCase()
</file context>

Suggested change

	if (!output.output || typeof output.output !== "string") return
	if (typeof output.output !== "string") output.output = ""

[code-yeongyu]

Closing in favor of #1756.

#1756 uses ?? "" (nullish coalescing) instead of early return, which is safer — early return here skips comment checks entirely for write/edit cases where output.output is undefined (cubic also flagged this as P2). #1756 also covers the atlas hook fix and has more comprehensive tests (101-line task-resume-info suite). cubic gave it 5/5 + auto-approve vs 4/5 here.