fix(core): return content-only Code Mode results (#41954)

Co-authored-by: Aiden Cline <rekram1-node@users.noreply.github.com>

packages/core/src/codemode/tool.ts

@@ -75,7 +75,9 @@ export const create = (
               const outputFileParts = outputFiles(content)
               if (outputFileParts.length > 0)
                 yield* Ref.update(files, (items) => [...items, { index, files: outputFileParts }])
-              return executed.output
+              if (executed.output !== undefined) return executed.output
+              const text = content.flatMap((part) => (part.type === "text" ? [part.text] : [])).join("\n")
+              return text === "" ? null : text
             }),
           {
             onToolCallStart: ({ index, name, input }) => {
@@ -155,7 +157,7 @@ function runtime(
     tools[path] = Tool.make({
       description: child.description,
       input: child.inputSchema,
-      output: child.outputSchema,
+      output: child.outputSchema ?? Schema.NullOr(Schema.String),
       execute: (input) => executeTool(name, registration, input),
     })
   }

packages/core/test/mcp.test.ts

@@ -248,6 +248,12 @@ const mcp = Layer.mock(MCP.Service, {
           required: ["ok"],
         },
       }),
+      new MCP.Tool({
+        server: MCP.ServerName.make("demo"),
+        name: "status",
+        description: "Status",
+        inputSchema: { type: "object", properties: {} },
+      }),
       new MCP.Tool({
         server: MCP.ServerName.make("direct"),
         name: "lookup",
@@ -290,6 +296,13 @@ const mcp = Layer.mock(MCP.Service, {
             { type: "media", data: "aGVsbG8=", mimeType: "image/png" },
           ],
         })
+      if (input.name === "status")
+        return new MCP.ToolResult({
+          server: MCP.ServerName.make(input.server),
+          tool: input.name,
+          isError: false,
+          content: [{ type: "text", text: "hello" }],
+        })
       return new MCP.ToolResult({
         server: MCP.ServerName.make(input.server),
         tool: input.name,
@@ -984,6 +997,31 @@ it.effect("advertises MCP output schemas to Code Mode", () =>
   }),
 )
 
+it.effect("returns content-only MCP results through Code Mode", () =>
+  Effect.gen(function* () {
+    assertion = yield* Deferred.make<Permission.AssertInput>()
+    decision = Effect.void
+    const registry = yield* Tool.Service
+    const toolSet = yield* waitForCodeModeTool(registry, "demo.status")
+
+    const execution = yield* toolSet.execute({
+      sessionID: Session.ID.make("ses_mcp_content_only"),
+      ...toolIdentity,
+      call: {
+        type: "tool-call",
+        id: "call_mcp_content_only",
+        name: "execute",
+        input: { code: "return await tools.demo.status({})" },
+      },
+    })
+
+    expect(execution).toMatchObject({
+      output: { output: "hello", toolCalls: [{ tool: "demo.status", status: "completed" }] },
+      content: [{ type: "text", text: "hello" }],
+    })
+  }),
+)
+
 it.effect("advertises MCP tools directly when Code Mode is disabled for the server", () =>
   Effect.gen(function* () {
     const registry = yield* Tool.Service

packages/core/test/plugin/promise.test.ts

@@ -393,4 +393,45 @@ describe("fromPromise", () => {
       expect(progress).toEqual([{ phase: "greeting" }])
     }),
   )
+
+  it.effect("returns content-only plugin results through Code Mode", () =>
+    Effect.gen(function* () {
+      const plugins = yield* Plugin.Service
+      const registry = yield* Tool.Service
+      const host = yield* PluginHost.make(plugins)
+      const promisePlugin = define({
+        id: "content-only-tool",
+        setup: async (ctx) => {
+          await ctx.tool.transform((tools) => {
+            tools.add({
+              name: "demo_status",
+              description: "Returns a status string",
+              input: Schema.Struct({}),
+              execute: async () => ({ content: [{ type: "text", text: "hello" }] }),
+              options: { codemode: true },
+            })
+          })
+        },
+      })
+
+      yield* PluginPromise.fromPromise(promisePlugin).effect(host)
+
+      const toolSet = yield* registry.snapshot()
+      const throughCodeMode = yield* toolSet.execute({
+        sessionID: Session.ID.make("ses_content_only_tool"),
+        agent: Agent.ID.make("build"),
+        messageID: SessionMessage.ID.make("msg_content_only_tool"),
+        call: {
+          type: "tool-call",
+          id: "call_content_only_tool",
+          name: "execute",
+          input: { code: "return await tools.demo_status({})" },
+        },
+      })
+      expect(throughCodeMode).toMatchObject({
+        output: { output: "hello", toolCalls: [{ tool: "demo_status", status: "completed" }] },
+        content: [{ type: "text", text: "hello" }],
+      })
+    }),
+  )
 })