Return artifact source from show-artifact

.changeset/artifact-source-text.md

@@ -0,0 +1,5 @@
+---
+"executor": patch
+---
+
+**Fix: `show-artifact` now returns the saved component source to MCP clients that cannot render Apps.** Agents can read the current source and make targeted edits instead of receiving only a link to the artifact.

e2e/scenarios/artifacts.test.ts

@@ -187,6 +187,7 @@ scenario(
     const suffix = uniqueSuffix();
     const title = `Release Readiness ${suffix}`;
     const marker = `artifact-ok-${suffix}`;
+    const source = artifactSource(marker).trim();
 
     // Tracked so cleanup runs even when an assertion below fails.
     let artifactId: ArtifactId | undefined;
@@ -215,7 +216,7 @@ scenario(
       );
 
       const rendered = yield* session.call("create-artifact", {
-        code: artifactSource(marker),
+        code: source,
         title,
         description: "Whether the current release is ready to ship",
       });
@@ -552,6 +553,10 @@ scenario(
         String(structuredOf(shown).url ?? shown.text),
         "show-artifact delivers the same deep link for a non-Apps client",
       ).toContain(String(artifactId));
+      expect(
+        shown.text,
+        "show-artifact includes the current source in its text result for a non-Apps client",
+      ).toContain(`Source:\n\`\`\`tsx\n${source}\n\`\`\``);
     }).pipe(
       Effect.ensuring(
         Effect.suspend(() =>

packages/hosts/mcp/src/artifacts-tools.test.ts

@@ -182,6 +182,13 @@ const structuredOf = (result: Awaited<ReturnType<Client["callTool"]>>): Record<s
 const textOf = (result: Awaited<ReturnType<Client["callTool"]>>): string =>
   (result.content as Array<{ type: string; text: string }>)[0].text;
 
+/** Assert source is available through both MCP result channels. */
+const expectArtifactSource = (result: Awaited<ReturnType<Client["callTool"]>>, code: string) => {
+  expect(structuredOf(result).code).toBe(code);
+  expect(textOf(result)).toContain("Source:");
+  expect(textOf(result)).toContain(code);
+};
+
 const toolNames = async (client: Client): Promise<string[]> =>
   (await client.listTools()).tools.map((tool) => tool.name);
 
@@ -688,8 +695,11 @@ describe("MCP host — create-artifact", () => {
           url: "https://executor.test/artifacts/art_1",
           artifactId: "art_1",
         });
-        // The model needs to be told to hand the URL over.
+        // The model needs to be told to hand the URL over. Source is a
+        // show-artifact read, not part of the create confirmation.
         expect(textOf(result)).toContain("https://executor.test/artifacts/art_1");
+        expect(textOf(result)).not.toContain("Source:");
+        expect(structuredOf(result)).not.toHaveProperty("code");
         // Persistence is what makes the fallback possible at all.
         expect(store.calls).toHaveLength(1);
         expect(store.rows.get("art_1")?.code).toBe(COUNTER_CODE);
@@ -1285,6 +1295,10 @@ describe("MCP host — artifact retrieval", () => {
           code: COUNTER_CODE,
           artifactId: "art_1",
         });
+        // Apps-capable hosts still need the source on the text channel: a
+        // later restore or a client that starts advertising apps must not
+        // make `show-artifact` unusable for `edit-artifact`.
+        expectArtifactSource(shown, COUNTER_CODE);
       },
       { artifacts: store.port },
     );
@@ -1368,7 +1382,12 @@ describe("MCP host — artifact retrieval", () => {
           status: "fallback_url",
           url: "https://executor.test/artifacts/art_1",
           artifactId: "art_1",
+          code: COUNTER_CODE,
         });
+        // The URL instruction stays; the source rides after it so a text-only
+        // host can copy `oldText` for `edit-artifact` from this result.
+        expect(textOf(shown)).toContain("https://executor.test/artifacts/art_1");
+        expectArtifactSource(shown, COUNTER_CODE);
       },
       {
         artifacts: store.port,
@@ -1377,6 +1396,35 @@ describe("MCP host — artifact retrieval", () => {
     );
   });
 
+  it("returns show-artifact source when the client has no apps support and no web UI", async () => {
+    const store = makeArtifactStore();
+    await Effect.runPromise(
+      store.port.save({
+        title: "Saved earlier",
+        description: null,
+        code: COUNTER_CODE,
+      }),
+    );
+    await withClient(
+      makeStubEngine({}),
+      NO_APPS_CAPS,
+      async (client) => {
+        const shown = await client.callTool({
+          name: "show-artifact",
+          arguments: { id: "art_1" },
+        });
+        expect(structuredOf(shown)).toEqual({
+          status: "fallback_unavailable",
+          reason: "mcp_apps_unsupported",
+          artifactId: "art_1",
+          code: COUNTER_CODE,
+        });
+        expectArtifactSource(shown, COUNTER_CODE);
+      },
+      { artifacts: store.port },
+    );
+  });
+
   it("reports a miss as an error result rather than failing the tool call", async () => {
     const store = makeArtifactStore();
     await withClient(

packages/hosts/mcp/src/tool-server.ts

@@ -900,6 +900,8 @@ const startMarker = (name: string, attributes: Record<string, unknown>): Effect.
 // user as an inline widget when the client renders MCP Apps, and as a link into
 // the web app when it doesn't. Both carry `artifactId`, because either way the
 // artifact was saved and can be reopened later.
+// `show-artifact` returns source on both channels; create/edit only confirm
+// saves.
 
 const renderRejectedResult = (reason: string): McpToolResult => ({
   content: [{ type: "text", text: `create-artifact rejected: ${reason}` }],
@@ -983,6 +985,24 @@ const bindingUnresolvedResult = (input: {
   isError: true,
 });
 
+/** Format the stored source for the text result channel. */
+const artifactSourceText = (code: string): string => `Source:\n\`\`\`tsx\n${code}\n\`\`\``;
+
+/** Add source to both MCP result channels. */
+const withArtifactSource = (result: McpToolResult, code: string): McpToolResult => {
+  const source = artifactSourceText(code);
+  const content = result.content.map((block, index) =>
+    index === 0 && block.type === "text"
+      ? { type: "text" as const, text: `${block.text}\n\n${source}` }
+      : block,
+  );
+  return {
+    ...result,
+    content,
+    structuredContent: { ...result.structuredContent, code },
+  };
+};
+
 const renderedInAppResult = (input: {
   readonly code: string;
   readonly artifactId: string;
@@ -2042,11 +2062,14 @@ export const createExecutorMcpServer = <E extends Cause.YieldableError>(
           .pipe(Effect.catchCause(() => Effect.succeed(null)));
         if (!artifact) return artifactNotFoundResult(id);
         yield* notifyArtifactUsage("viewed");
-        return deliverArtifact({
-          code: artifact.code,
-          artifactId: artifact.id,
-          title: artifact.title,
-        });
+        return withArtifactSource(
+          deliverArtifact({
+            code: artifact.code,
+            artifactId: artifact.id,
+            title: artifact.title,
+          }),
+          artifact.code,
+        );
       }).pipe(
         Effect.withSpan("mcp.host.tool.show_artifact", {
           attributes: { "mcp.tool.name": "show-artifact", "mcp.artifact.id": id },
@@ -2245,7 +2268,7 @@ export const createExecutorMcpServer = <E extends Cause.YieldableError>(
             description: [
               "Re-render a saved UI artifact by id.",
               "Use `list-artifacts` first to find the id whose title or description matches what the user asked for.",
-              "Clients that cannot display MCP apps receive a link to the artifact instead.",
+              "Returns the artifact's current source. Clients that cannot display MCP apps also receive a link to the artifact; pass it to the user.",
             ].join("\n"),
             inputSchema: {
               id: z.string().trim().min(1).describe("The artifact id from `list-artifacts`."),