diff --git a/internal/workflow/loader_test.go b/internal/workflow/loader_test.go
index d3788fd..c101f90 100644
--- a/internal/workflow/loader_test.go
+++ b/internal/workflow/loader_test.go
@@ -3,6 +3,7 @@ package workflow //nolint:testpackage // needs access to unexported userHomeDir
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -247,6 +248,24 @@ func TestReadPrompt_embeddedFallback(t *testing.T) { //nolint:paralleltest // mu
 	}
 }
 
+func TestReadPrompt_embeddedImplementPromptMentionsGitHubClosingReference(t *testing.T) { //nolint:paralleltest // mutates package-level userHomeDir
+	tmp := t.TempDir()
+	userHomeDir = func() (string, error) { return tmp, nil }
+	t.Cleanup(func() { userHomeDir = os.UserHomeDir })
+
+	data, err := ReadPrompt(tmp, "prompts/implement.md")
+	if err != nil {
+		t.Fatalf("expected embedded fallback, got error: %v", err)
+	}
+	content := string(data)
+	if !strings.Contains(content, "Closes #<number>") {
+		t.Fatalf("expected implement prompt to mention GitHub closing reference, got %q", content)
+	}
+	if !strings.Contains(content, "Do not use cross-repository issue references") {
+		t.Fatalf("expected implement prompt to restrict cross-repository references, got %q", content)
+	}
+}
+
 func TestReadPrompt_notFound(t *testing.T) { //nolint:paralleltest // mutates package-level userHomeDir
 	tmp := t.TempDir()
 	userHomeDir = func() (string, error) { return tmp, nil }
diff --git a/internal/workflow/prompts/implement.md b/internal/workflow/prompts/implement.md
index 8696039..1615043 100644
--- a/internal/workflow/prompts/implement.md
+++ b/internal/workflow/prompts/implement.md
@@ -5,6 +5,8 @@ Read `.auto-pr/run-context.md`.
 Read the ticket details from the latest `fetch-ticket-data` artifact path listed in the `Latest State Artifacts` section of `.auto-pr/run-context.md`.
 Read the approved proposal from the latest `investigation` artifact path listed in the `Latest State Artifacts` section of `.auto-pr/run-context.md`.
 
+If the fetched ticket has a GitHub issue `URL:` for this repository, include `Closes #<number>` in the pull request body when you create the PR. Do not use cross-repository issue references for this workflow.
+
 If the `Feedback File` listed in `.auto-pr/run-context.md` exists, incorporate that feedback into your implementation.
 
 If a `Guidelines File` is listed in `.auto-pr/run-context.md`, read and follow those guidelines.