diff --git a/pkg/actionpins/actionpins_internal_test.go b/pkg/actionpins/actionpins_internal_test.go
index a483b1235d5..21cf09295a9 100644
--- a/pkg/actionpins/actionpins_internal_test.go
+++ b/pkg/actionpins/actionpins_internal_test.go
@@ -172,7 +172,8 @@ func TestFormatPinnedActionReference_PanicsWhenSHAIsEmpty(t *testing.T) {
 }
 
 func TestPinContextEmitOnce_InitializesAndDeduplicates(t *testing.T) {
-	t.Parallel()
+	// Not parallel: CaptureStderr swaps the global os.Stderr and is not safe
+	// to run concurrently with other tests that write to stderr.
 
 	ctx := &PinContext{}
 
@@ -953,7 +954,8 @@ func TestApplyContainerPinMapping(t *testing.T) {
 }
 
 func TestApplyContainerPinMapping_DeduplicatesInvalidWarnings(t *testing.T) {
-	t.Parallel()
+	// Not parallel: CaptureStderr swaps the global os.Stderr and is not safe
+	// to run concurrently with other tests that write to stderr.
 
 	ctx := &PinContext{
 		ContainerMappings: map[string]string{
diff --git a/pkg/cli/add_package_manifest.go b/pkg/cli/add_package_manifest.go
index 719a136b669..a31e3ec6199 100644
--- a/pkg/cli/add_package_manifest.go
+++ b/pkg/cli/add_package_manifest.go
@@ -11,6 +11,7 @@ import (
 	"github.com/goccy/go-yaml"
 
 	"github.com/github/gh-aw/pkg/constants"
+	"github.com/github/gh-aw/pkg/errorutil"
 	"github.com/github/gh-aw/pkg/logger"
 	"github.com/github/gh-aw/pkg/parser"
 	"github.com/github/gh-aw/pkg/semverutil"
@@ -1005,20 +1006,12 @@ func listRepositoryPackageDirSubdirsForHost(ctx context.Context, owner, repo, re
 }
 
 func normalizeRepositoryPackageRemoteError(err error) error {
-	if err == nil || !isRepositoryPackageRemoteNotFound(err) {
+	if err == nil || !errorutil.IsNotFoundError(err) {
 		return err
 	}
 	return packageRemoteNotFoundError{cause: err}
 }
 
-func isRepositoryPackageRemoteNotFound(err error) bool {
-	if err == nil {
-		return false
-	}
-	errText := strings.ToLower(err.Error())
-	return strings.Contains(errText, "404") || strings.Contains(errText, "not found")
-}
-
 func resolveRepositoryPackageDefaultBranch(ctx context.Context, repoSlug, host string) (string, error) {
 	args := []string{"api", "/repos/" + repoSlug, "--jq", ".default_branch"}
 	var output []byte
diff --git a/pkg/cli/branch_file_reader.go b/pkg/cli/branch_file_reader.go
index 7bbf8018002..b26698d5793 100644
--- a/pkg/cli/branch_file_reader.go
+++ b/pkg/cli/branch_file_reader.go
@@ -8,6 +8,7 @@ import (
 	"os"
 	"strings"
 
+	"github.com/github/gh-aw/pkg/errorutil"
 	"github.com/github/gh-aw/pkg/logger"
 	"github.com/github/gh-aw/pkg/workflow"
 )
@@ -33,7 +34,7 @@ func readRemoteRepoBranchFileContext(ctx context.Context, repoOverride, branchNa
 	cmd := workflow.ExecGHContext(ctx, args...)
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		if isRemoteFileNotFoundOutput(string(out)) {
+		if errorutil.IsNotFoundOutput(string(out)) {
 			branchFileReaderLog.Printf("Remote file not found: path=%s, branch=%s", filePath, branchName)
 			return nil, os.ErrNotExist
 		}
@@ -55,8 +56,3 @@ func readRemoteRepoBranchFileContext(ctx context.Context, repoOverride, branchNa
 func isRemoteFileNotFound(err error) bool {
 	return errors.Is(err, os.ErrNotExist)
 }
-
-func isRemoteFileNotFoundOutput(output string) bool {
-	s := strings.ToLower(output)
-	return strings.Contains(s, "404") || strings.Contains(s, "not found")
-}
diff --git a/pkg/cli/checks_command.go b/pkg/cli/checks_command.go
index 07056a1eebc..ab729e9fa8c 100644
--- a/pkg/cli/checks_command.go
+++ b/pkg/cli/checks_command.go
@@ -10,6 +10,7 @@ import (
 
 	"github.com/github/gh-aw/pkg/console"
 	"github.com/github/gh-aw/pkg/constants"
+	"github.com/github/gh-aw/pkg/errorutil"
 	"github.com/github/gh-aw/pkg/logger"
 	"github.com/github/gh-aw/pkg/sliceutil"
 	"github.com/github/gh-aw/pkg/workflow"
@@ -240,7 +241,7 @@ func classifyGHAPIError(exitCode int, stderr string, prNumber string, repo strin
 	lower := strings.ToLower(stderr)
 
 	switch {
-	case strings.Contains(lower, "404") || strings.Contains(lower, "not found"):
+	case errorutil.IsNotFoundOutput(stderr):
 		repoHint := "the current repository"
 		if repo != "" {
 			repoHint = repo
diff --git a/pkg/cli/evals_branch.go b/pkg/cli/evals_branch.go
index 13f4a45466b..bee2bef3404 100644
--- a/pkg/cli/evals_branch.go
+++ b/pkg/cli/evals_branch.go
@@ -11,6 +11,7 @@ import (
 	"strings"
 
 	"github.com/github/gh-aw/pkg/constants"
+	"github.com/github/gh-aw/pkg/errorutil"
 	"github.com/github/gh-aw/pkg/parser"
 	"github.com/github/gh-aw/pkg/stringutil"
 	"github.com/github/gh-aw/pkg/workflow"
@@ -141,7 +142,7 @@ func resolveRunStateBranchRef(ctx context.Context, repoOverride, branchName stri
 		cmd := workflow.ExecGHContext(ctx, args...)
 		out, err := cmd.CombinedOutput()
 		if err != nil {
-			if isRemoteFileNotFoundOutput(string(out)) {
+			if errorutil.IsNotFoundOutput(string(out)) {
 				return "", os.ErrNotExist
 			}
 			return "", fmt.Errorf("failed to list commits for state branch %s: %w", branchName, err)
diff --git a/pkg/cli/experiments_command.go b/pkg/cli/experiments_command.go
index c03299d92c6..10f45d90724 100644
--- a/pkg/cli/experiments_command.go
+++ b/pkg/cli/experiments_command.go
@@ -17,6 +17,7 @@ import (
 
 	"github.com/github/gh-aw/pkg/console"
 	"github.com/github/gh-aw/pkg/constants"
+	"github.com/github/gh-aw/pkg/errorutil"
 	"github.com/github/gh-aw/pkg/logger"
 	"github.com/github/gh-aw/pkg/parser"
 	"github.com/github/gh-aw/pkg/setutil"
@@ -715,7 +716,7 @@ func fetchRemoteExperimentDetails(repoOverride, branchName, workflowID string) (
 		var exitErr *exec.ExitError
 		if errors.As(err, &exitErr) {
 			stderr := strings.TrimSpace(string(exitErr.Stderr))
-			if strings.Contains(stderr, "404") || strings.Contains(stderr, "not found") {
+			if errorutil.IsNotFoundOutput(stderr) {
 				return nil, fmt.Errorf("experiment %q not found in %s", workflowID, repoOverride)
 			}
 			return nil, fmt.Errorf("failed to fetch experiment branch (exit %d): %s", exitErr.ExitCode(), stderr)
diff --git a/pkg/cli/experiments_command_test.go b/pkg/cli/experiments_command_test.go
index d5c9976ba5a..e145bdbb0ca 100644
--- a/pkg/cli/experiments_command_test.go
+++ b/pkg/cli/experiments_command_test.go
@@ -4,12 +4,25 @@ package cli
 
 import (
 	"encoding/json"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+func TestFetchRemoteExperimentDetailsClassifiesTitleCaseNotFound(t *testing.T) {
+	fakeBinDir := t.TempDir()
+	fakeGH := filepath.Join(fakeBinDir, "gh")
+	require.NoError(t, os.WriteFile(fakeGH, []byte("#!/bin/sh\necho 'HTTP 404: Not Found' >&2\nexit 1\n"), 0o755))
+	t.Setenv("PATH", fakeBinDir+string(os.PathListSeparator)+os.Getenv("PATH"))
+
+	_, err := fetchRemoteExperimentDetails("octo/repo", "experiments/missing", "missing")
+
+	require.EqualError(t, err, `experiment "missing" not found in octo/repo`)
+}
+
 func TestExtractExperimentName(t *testing.T) {
 	tests := []struct {
 		name     string
diff --git a/pkg/cli/logs_download.go b/pkg/cli/logs_download.go
index 4e07ecbc1f7..2df2b61379c 100644
--- a/pkg/cli/logs_download.go
+++ b/pkg/cli/logs_download.go
@@ -101,7 +101,7 @@ func downloadWorkflowRunLogs(ctx context.Context, runID int64, outputDir string,
 		// Check both the Go error (via errorutil.IsNotFoundError) and the raw CLI output,
 		// as the gh CLI may write "not found" to stdout without reflecting it in the error.
 		// Also treat HTTP 410 Gone as non-critical (logs may be expired).
-		if errorutil.IsNotFoundError(err) || errorutil.IsNotFoundError(errors.New(string(output))) || errorutil.IsGoneError(err) {
+		if errorutil.IsNotFoundError(err) || errorutil.IsNotFoundOutput(string(output)) || errorutil.IsGoneError(err) {
 			if verbose {
 				fmt.Fprintln(os.Stderr, console.FormatWarningMessage(fmt.Sprintf("No logs found for run %d (may be expired or unavailable)", runID)))
 			}
diff --git a/pkg/cli/setup_repository.go b/pkg/cli/setup_repository.go
index 14f19d4c9c5..01bf149785b 100644
--- a/pkg/cli/setup_repository.go
+++ b/pkg/cli/setup_repository.go
@@ -11,6 +11,7 @@ import (
 
 	"github.com/github/gh-aw/pkg/console"
 	"github.com/github/gh-aw/pkg/ctxutil"
+	"github.com/github/gh-aw/pkg/errorutil"
 	"github.com/github/gh-aw/pkg/gitutil"
 	"github.com/github/gh-aw/pkg/logger"
 	"github.com/github/gh-aw/pkg/workflow"
@@ -104,8 +105,9 @@ func checkSetupRepositoryExists(ctx context.Context, repo string) (bool, error)
 		return strings.TrimSpace(string(output)) != "", nil
 	}
 
-	message := strings.ToLower(string(output))
-	if strings.Contains(message, "could not resolve to a repository") || strings.Contains(message, "http 404") || strings.Contains(message, "not found") {
+	outputStr := string(output)
+	message := strings.ToLower(outputStr)
+	if strings.Contains(message, "could not resolve to a repository") || errorutil.IsNotFoundOutput(outputStr) {
 		return false, nil
 	}
 	return false, fmt.Errorf("failed to check repository %s: %w", repo, err)
diff --git a/pkg/cli/workflow_run_metadata.go b/pkg/cli/workflow_run_metadata.go
index 530164040b6..86f8e20a06c 100644
--- a/pkg/cli/workflow_run_metadata.go
+++ b/pkg/cli/workflow_run_metadata.go
@@ -3,7 +3,6 @@ package cli
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -53,7 +52,7 @@ func buildWorkflowRunMetadataArgs(runID int64, owner, repo, hostname string) []s
 func classifyWorkflowRunMetadataError(runID int64, err error, output []byte) error {
 	outputStr := string(output)
 	if errorutil.IsNotFoundError(err) ||
-		errorutil.IsNotFoundError(errors.New(outputStr)) ||
+		errorutil.IsNotFoundOutput(outputStr) ||
 		strings.Contains(outputStr, "Could not resolve") {
 		return fmt.Errorf("workflow run %d not found. Please verify the run ID is correct and that you have access to the repository", runID)
 	}
diff --git a/pkg/errorutil/errors.go b/pkg/errorutil/errors.go
index afc34f5d95c..dd98130138b 100644
--- a/pkg/errorutil/errors.go
+++ b/pkg/errorutil/errors.go
@@ -23,6 +23,17 @@ func IsNotFoundError(err error) bool {
 	return matched
 }
 
+// IsNotFoundOutput reports whether output represents an HTTP 404 / "not found" response.
+// The check is case-insensitive and matches both the numeric literal "404" and
+// the phrase "not found".
+func IsNotFoundOutput(output string) bool {
+	matched := containsSubstring(output, "404", "not found")
+	if matched {
+		errorutilLog.Printf("Classified output as not-found (404): %s", output)
+	}
+	return matched
+}
+
 // IsForbiddenError reports whether err represents an HTTP 403 / "forbidden" response.
 // It returns false when err is nil.
 // The check is case-insensitive and only matches HTTP-style 403 patterns such as
@@ -56,7 +67,11 @@ func containsErrorSubstring(err error, substrings ...string) bool {
 	if err == nil {
 		return false
 	}
-	msg := strings.ToLower(err.Error())
+	return containsSubstring(err.Error(), substrings...)
+}
+
+func containsSubstring(value string, substrings ...string) bool {
+	msg := strings.ToLower(value)
 	for _, substring := range substrings {
 		if strings.Contains(msg, substring) {
 			return true
diff --git a/pkg/errorutil/errors_test.go b/pkg/errorutil/errors_test.go
index 03ccab5465f..193028e9841 100644
--- a/pkg/errorutil/errors_test.go
+++ b/pkg/errorutil/errors_test.go
@@ -45,6 +45,26 @@ func TestIsNotFoundError(t *testing.T) {
 	}
 }
 
+func TestIsNotFoundOutput(t *testing.T) {
+	tests := []struct {
+		name   string
+		output string
+		want   bool
+	}{
+		{name: "empty output", output: "", want: false},
+		{name: "404 numeric literal", output: "HTTP 404: Not Found", want: true},
+		{name: "title case not found", output: "GraphQL: Not Found", want: true},
+		{name: "uppercase not found", output: "RESOURCE NOT FOUND", want: true},
+		{name: "generic output", output: "something went wrong", want: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, errorutil.IsNotFoundOutput(tt.output))
+		})
+	}
+}
+
 func TestIsForbiddenError(t *testing.T) {
 	tests := []struct {
 		name string