Report error when API silently fails to add review comment

pkg/github/pullrequests.go

@@ -1518,6 +1518,14 @@ func AddCommentToPendingReview(getGQLClient GetGQLClientFn, t translations.Trans
 				return mcp.NewToolResultError(err.Error()), nil
 			}
 
+			if addPullRequestReviewThreadMutation.AddPullRequestReviewThread.Thread.ID == nil {
+				return mcp.NewToolResultError(`Failed to add comment to pending review. Possible reasons:
+	- The line number doesn't exist in the pull request diff
+	- The file path is incorrect
+	- The side (LEFT/RIGHT) is invalid for the specified line
+`), nil
+			}
+
 			// Return nothing interesting, just indicate success for the time being.
 			// In future, we may want to return the review ID, but for the moment, we're not leaking
 			// API implementation details to the LLM.

pkg/github/pullrequests_test.go

@@ -2555,10 +2555,75 @@ func TestAddPullRequestReviewCommentToPendingReview(t *testing.T) {
 						PullRequestReviewID: githubv4.NewID("PR_kwDODKw3uc6WYN1T"),
 					},
 					nil,
-					githubv4mock.DataResponse(map[string]any{}),
+					githubv4mock.DataResponse(map[string]any{
+						"addPullRequestReviewThread": map[string]any{
+							"thread": map[string]any{
+								"id": "MDEyOlB1bGxSZXF1ZXN0UmV2aWV3VGhyZWFkMTIzNDU2",
+							},
+						},
+					}),
 				),
 			),
 		},
+		{
+			name: "thread ID is nil - invalid line number",
+			requestArgs: map[string]any{
+				"owner":       "owner",
+				"repo":        "repo",
+				"pullNumber":  float64(42),
+				"path":        "file.go",
+				"body":        "Comment on non-existent line",
+				"subjectType": "LINE",
+				"line":        float64(999),
+				"side":        "RIGHT",
+			},
+			mockedClient: githubv4mock.NewMockedHTTPClient(
+				viewerQuery("williammartin"),
+				getLatestPendingReviewQuery(getLatestPendingReviewQueryParams{
+					author: "williammartin",
+					owner:  "owner",
+					repo:   "repo",
+					prNum:  42,
+
+					reviews: []getLatestPendingReviewQueryReview{
+						{
+							id:    "PR_kwDODKw3uc6WYN1T",
+							state: "PENDING",
+							url:   "https://github.com/owner/repo/pull/42",
+						},
+					},
+				}),
+				githubv4mock.NewMutationMatcher(
+					struct {
+						AddPullRequestReviewThread struct {
+							Thread struct {
+								ID githubv4.ID
+							}
+						} `graphql:"addPullRequestReviewThread(input: $input)"`
+					}{},
+					githubv4.AddPullRequestReviewThreadInput{
+						Path:                githubv4.String("file.go"),
+						Body:                githubv4.String("Comment on non-existent line"),
+						SubjectType:         githubv4mock.Ptr(githubv4.PullRequestReviewThreadSubjectTypeLine),
+						Line:                githubv4.NewInt(999),
+						Side:                githubv4mock.Ptr(githubv4.DiffSideRight),
+						StartLine:           nil,
+						StartSide:           nil,
+						PullRequestReviewID: githubv4.NewID("PR_kwDODKw3uc6WYN1T"),
+					},
+					nil,
+					githubv4mock.DataResponse(map[string]any{
+						"addPullRequestReviewThread": map[string]any{
+							"thread": map[string]any{
+								"id": nil,
+							},
+						},
+					}),
+				),
+			),
+			expectToolError:    true,
+			expectedToolErrMsg: "Failed to add comment to pending review",
+		},
 	}
 
 	for _, tc := range tests {