fix(x): surface coding tool errors

apps/x/packages/core/src/application/assistant/skills/code-with-agents/skill.ts

@@ -79,7 +79,7 @@ After \`code_agent_run\` returns:
 - Pass through the agent's \`summary\` as-is. Do not rewrite it.
 - Refer to file paths as plain text. Do NOT use \`\`\`file:path\`\`\` reference blocks. (This overrides the global "always wrap paths in filepath blocks" rule — for code-mode output, plain text.)
 - Only add your own explanation if it failed:
-  - \`success: false\` with a message — surface the message. If it mentions the agent isn't installed or signed in, tell the user to install or sign in via **Settings → Code Mode**.
+  - A tool error with a message — surface the message. If it mentions the agent isn't installed or signed in, tell the user to install or sign in via **Settings → Code Mode**.
   - \`stopReason: "cancelled"\` — the run was stopped; acknowledge briefly and ask if they want to continue.
 
 ---

apps/x/packages/core/src/application/lib/builtin-tools.test.ts

@@ -0,0 +1,54 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import container from "../../di/container.js";
+import { InMemoryAbortRegistry } from "../../runs/abort-registry.js";
+import { BuiltinTools } from "./builtin-tools.js";
+import type { ToolContext } from "./exec-tool.js";
+
+function context(signal: AbortSignal): ToolContext {
+    return {
+        runId: "turn-1",
+        toolCallId: "tool-1",
+        signal,
+        abortRegistry: new InMemoryAbortRegistry(),
+        publish: async () => {},
+        codePolicy: "ask",
+    };
+}
+
+function mockCodeServices(runPrompt: () => Promise<never>): void {
+    vi.spyOn(container, "resolve").mockImplementation(((name: string) => {
+        if (name === "codeModeManager") return { runPrompt };
+        if (name === "codePermissionRegistry") {
+            return { cancelRun: vi.fn(), request: vi.fn() };
+        }
+        throw new Error(`Unexpected dependency: ${name}`);
+    }) as typeof container.resolve);
+}
+
+describe("code_agent_run", () => {
+    afterEach(() => vi.restoreAllMocks());
+
+    it("throws genuine coding-agent failures for the runtime to mark as errors", async () => {
+        mockCodeServices(async () => {
+            throw new Error("spawn Electron ENOENT");
+        });
+
+        await expect(BuiltinTools.code_agent_run.execute(
+            { agent: "codex", cwd: "/repo", prompt: "Fix it" },
+            context(new AbortController().signal),
+        )).rejects.toThrow("Coding agent failed: spawn Electron ENOENT");
+    });
+
+    it("returns an ordinary cancellation result when the turn was aborted", async () => {
+        const controller = new AbortController();
+        controller.abort();
+        mockCodeServices(async () => {
+            throw new Error("cancelled");
+        });
+
+        await expect(BuiltinTools.code_agent_run.execute(
+            { agent: "codex", cwd: "/repo", prompt: "Fix it" },
+            context(controller.signal),
+        )).resolves.toMatchObject({ success: false, stopReason: "cancelled" });
+    });
+});

apps/x/packages/core/src/application/lib/builtin-tools.ts

@@ -868,7 +868,7 @@ export const BuiltinTools: z.infer<typeof BuiltinToolsSchema> = {
         }),
         execute: async ({ agent, cwd, prompt }: { agent: 'claude' | 'codex', cwd: string, prompt: string }, ctx?: ToolContext) => {
             if (!ctx) {
-                return { success: false, message: 'code_agent_run requires run context (runId / streaming).' };
+                throw new Error('code_agent_run requires run context (runId / streaming).');
             }
             // The composer chip is the source of truth for the agent. The model's `agent`
             // argument is only a fallback for the ask-human flow (code mode not active, no
@@ -955,10 +955,7 @@ export const BuiltinTools: z.infer<typeof BuiltinToolsSchema> = {
                         changedFiles: [...changedFiles],
                     };
                 }
-                return {
-                    success: false,
-                    message: `Coding agent failed: ${error instanceof Error ? error.message : String(error)}`,
-                };
+                throw new Error(`Coding agent failed: ${error instanceof Error ? error.message : String(error)}`);
             } finally {
                 ctx.signal.removeEventListener('abort', onAbort);
             }