feat: display detailed error messages when prepare step fails

- Capture prepare step errors in action.yml (up to 2000 chars)
- Add error details to comment update with collapsible section
- Handle both prepare and Claude execution failures separately
- Add test coverage for error detail display

This helps users debug issues like git errors, permission problems,
and branch creation failures more easily.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

action.yml

@@ -81,7 +81,26 @@ runs:
       id: prepare
       shell: bash
       run: |
-        bun run ${{ github.action_path }}/src/entrypoints/prepare.ts
+        set +e
+        # Create a temporary file to capture both stdout and stderr
+        TEMP_OUTPUT=$(mktemp)
+        bun run ${{ github.action_path }}/src/entrypoints/prepare.ts 2>&1 | tee "$TEMP_OUTPUT"
+        PREPARE_EXIT_CODE=${PIPESTATUS[0]}
+
+        # If the command failed, save the error output
+        if [ $PREPARE_EXIT_CODE -ne 0 ]; then
+          # Extract last 50 lines of output as error details
+          ERROR_DETAILS=$(tail -n 50 "$TEMP_OUTPUT")
+          echo "prepare_error<<EOF" >> $GITHUB_OUTPUT
+          echo "$ERROR_DETAILS" >> $GITHUB_OUTPUT
+          echo "EOF" >> $GITHUB_OUTPUT
+        fi
+
+        # Clean up temp file
+        rm -f "$TEMP_OUTPUT"
+
+        # Exit with the original exit code
+        exit $PREPARE_EXIT_CODE
       env:
         TRIGGER_PHRASE: ${{ inputs.trigger_phrase }}
         ASSIGNEE_TRIGGER: ${{ inputs.assignee_trigger }}
@@ -147,6 +166,8 @@ runs:
         CLAUDE_SUCCESS: ${{ steps.claude-code.outputs.conclusion == 'success' }}
         OUTPUT_FILE: ${{ steps.claude-code.outputs.execution_file || '' }}
         TRIGGER_USERNAME: ${{ github.event.comment.user.login || github.event.issue.user.login || github.event.pull_request.user.login || github.event.sender.login || github.triggering_actor || github.actor || '' }}
+        PREPARE_SUCCESS: ${{ steps.prepare.outcome == 'success' }}
+        PREPARE_ERROR: ${{ steps.prepare.outputs.prepare_error || '' }}
 
     - name: Display Claude Code Report
       if: steps.prepare.outputs.contains_trigger == 'true' && steps.claude-code.outputs.execution_file != ''

src/entrypoints/update-comment-link.ts

@@ -145,7 +145,16 @@ async function run() {
       duration_api_ms?: number;
     } | null = null;
     let actionFailed = false;
+    let errorDetails: string | undefined;
 
+    // First check if prepare step failed
+    const prepareSuccess = process.env.PREPARE_SUCCESS !== "false";
+    const prepareError = process.env.PREPARE_ERROR;
+
+    if (!prepareSuccess && prepareError) {
+      actionFailed = true;
+      errorDetails = prepareError;
+    } else {
       // Check for existence of output file and parse it if available
       try {
         const outputFile = process.env.OUTPUT_FILE;
@@ -170,7 +179,7 @@ async function run() {
           }
         }
 
-      // Check if the action failed by looking at the exit code or error marker
+        // Check if the Claude action failed
         const claudeSuccess = process.env.CLAUDE_SUCCESS !== "false";
         actionFailed = !claudeSuccess;
       } catch (error) {
@@ -178,6 +187,7 @@ async function run() {
         // If we can't read the file, check for any failure markers
         actionFailed = process.env.CLAUDE_SUCCESS === "false";
       }
+    }
 
     // Prepare input for updateCommentBody function
     const commentInput: CommentUpdateInput = {
@@ -189,6 +199,7 @@ async function run() {
       prLink,
       branchName: shouldDeleteBranch ? undefined : claudeBranch,
       triggerUsername,
+      errorDetails,
     };
 
     const updatedBody = updateCommentBody(commentInput);

src/github/operations/comment-logic.ts

@@ -15,6 +15,7 @@ export type CommentUpdateInput = {
   prLink?: string;
   branchName?: string;
   triggerUsername?: string;
+  errorDetails?: string;
 };
 
 export function ensureProperlyEncodedUrl(url: string): string | null {
@@ -75,6 +76,7 @@ export function updateCommentBody(input: CommentUpdateInput): string {
     actionFailed,
     branchName,
     triggerUsername,
+    errorDetails,
   } = input;
 
   // Extract content from the original comment body
@@ -177,7 +179,14 @@ export function updateCommentBody(input: CommentUpdateInput): string {
   }
 
   // Build the new body with blank line between header and separator
-  let newBody = `${header}${links}\n\n---\n`;
+  let newBody = `${header}${links}`;
+
+  // Add error details if available
+  if (actionFailed && errorDetails) {
+    newBody += `\n\n<details>\n<summary>Error details</summary>\n\n\`\`\`\n${errorDetails}\n\`\`\`\n\n</details>`;
+  }
+
+  newBody += `\n\n---\n`;
 
   // Clean up the body content
   // Remove any existing View job run, branch links from the bottom

test/comment-logic.test.ts

@@ -39,6 +39,28 @@ describe("updateCommentBody", () => {
       expect(result).toContain("**Claude encountered an error after 45s**");
     });
 
+    it("includes error details when provided", () => {
+      const input = {
+        ...baseInput,
+        currentBody: "Claude Code is working...",
+        actionFailed: true,
+        executionDetails: { duration_ms: 45000 },
+        errorDetails:
+          "fatal: not a git repository (or any of the parent directories): .git",
+      };
+
+      const result = updateCommentBody(input);
+      expect(result).toContain("**Claude encountered an error after 45s**");
+      expect(result).toContain("[View job]");
+      expect(result).toContain("<details>");
+      expect(result).toContain("<summary>Error details</summary>");
+      expect(result).toContain("fatal: not a git repository");
+      // Ensure error details come after the header/links
+      const errorIndex = result.indexOf("<details>");
+      const headerIndex = result.indexOf("**Claude encountered an error");
+      expect(errorIndex).toBeGreaterThan(headerIndex);
+    });
+
     it("handles username extraction from content when not provided", () => {
       const input = {
         ...baseInput,