Warn when an agent plugin prompt file is missing

src/plugins/agent-plugins.test.ts

@@ -176,4 +176,22 @@ describe("resolveAgentPluginProfiles", () => {
       ),
     ).toBe(true);
   });
+
+  test("warns once when a prompt file is missing and still loads the profile", async () => {
+    const { mod, config } = agentModule("p1", [
+      { id: "scout", systemPromptPath: "prompts/does-not-exist.md" },
+    ]);
+    mod.dir = "/tmp/wt-cl-6724-missing-prompt-dir";
+    const warnings: string[] = [];
+    const profiles = await resolveAgentPluginProfiles([mod], config, (msg) =>
+      warnings.push(msg),
+    );
+    expect(profiles.length).toBe(1);
+    expect(defined(profiles[0]).systemPromptRole).toBeUndefined();
+    expect(warnings.length).toBe(1);
+    expect(warnings[0]).toContain('"p1"');
+    expect(warnings[0]).toContain('"scout"');
+    expect(warnings[0]).toContain("prompts/does-not-exist.md");
+    expect(defined(warnings[0])).toMatch(/unreadable|missing/i);
+  });
 });

src/plugins/agent-plugins.ts

@@ -90,14 +90,18 @@ export async function resolveAgentPluginProfiles(
         profile.systemPromptRole === undefined &&
         mod.dir !== undefined
       ) {
+        const promptPath = join(mod.dir, profile.systemPromptPath);
         try {
-          const promptRaw = await readFile(
-            join(mod.dir, profile.systemPromptPath),
-            "utf8",
-          );
+          const promptRaw = await readFile(promptPath, "utf8");
           profile.systemPromptRole = promptRaw.trim();
-        } catch {
-          // Missing prompt file is non-fatal — the profile loads without a role.
+        } catch (err) {
+          const reason =
+            (err instanceof Error ? err.message : String(err))
+              .split("\n")[0]
+              ?.trim() || "unknown error";
+          onWarning(
+            `plugin "${mod.manifest.id}" agent "${profile.id}" systemPromptPath "${profile.systemPromptPath}" unreadable (${promptPath}): ${reason}`,
+          );
         }
       }
       // Provenance for search_agents: Claude marketplace installs stamp