feat(core): parse JSON text results from MCP tools without an output schema (#48357)

packages/core/src/tool/mcp.ts

@@ -99,8 +99,19 @@ export const layer = Layer.effect(
                           },
                     )
                     const text = content.flatMap((part) => (part.type === "text" ? [part.text] : [])).join("\n")
+                    const output = () => {
+                      if (result.structured !== undefined) return result.structured
+                      if (text === "") return null
+                      // Agents assume JSON returned as text is already an object, so parse it when the server declares no schema.
+                      if (tool.outputSchema === undefined && (text.startsWith("{") || text.startsWith("["))) {
+                        try {
+                          return JSON.parse(text)
+                        } catch {}
+                      }
+                      return text
+                    }
                     return {
-                      output: result.structured ?? (text === "" ? null : text),
+                      output: output(),
                       ...(content.length === 0 ? {} : { content }),
                     }
                   }).pipe(

packages/core/test/mcp.test.ts

@@ -324,6 +324,32 @@ const mcp = Layer.mock(Mcp.Service, {
         description: "Status",
         inputSchema: { type: "object", properties: {} },
       }),
+      new Mcp.Tool({
+        server: Mcp.ServerName.make("demo"),
+        name: "issues",
+        description: "Returns JSON as text",
+        inputSchema: { type: "object", properties: {} },
+      }),
+      new Mcp.Tool({
+        server: Mcp.ServerName.make("demo"),
+        name: "count",
+        description: "Returns a number as text",
+        inputSchema: { type: "object", properties: {} },
+      }),
+      new Mcp.Tool({
+        server: Mcp.ServerName.make("demo"),
+        name: "typed",
+        description: "Declares a string output and returns JSON as text",
+        inputSchema: { type: "object", properties: {} },
+        outputSchema: { type: "string" },
+      }),
+      new Mcp.Tool({
+        server: Mcp.ServerName.make("direct"),
+        name: "issues",
+        codemode: false,
+        description: "Returns JSON as text",
+        inputSchema: { type: "object", properties: {} },
+      }),
       new Mcp.Tool({
         server: Mcp.ServerName.make("direct"),
         name: "lookup",
@@ -374,6 +400,20 @@ const mcp = Layer.mock(Mcp.Service, {
           isError: false,
           content: [{ type: "text", text: "hello" }],
         })
+      if (input.name === "issues" || input.name === "typed")
+        return new Mcp.ToolResult({
+          server: Mcp.ServerName.make(input.server),
+          tool: input.name,
+          isError: false,
+          content: [{ type: "text", text: '{"issues":[{"id":1}]}' }],
+        })
+      if (input.name === "count")
+        return new Mcp.ToolResult({
+          server: Mcp.ServerName.make(input.server),
+          tool: input.name,
+          isError: false,
+          content: [{ type: "text", text: "42" }],
+        })
       return new Mcp.ToolResult({
         server: Mcp.ServerName.make(input.server),
         tool: input.name,
@@ -1943,6 +1983,7 @@ it.effect("advertises MCP output schemas to Code Mode", () =>
 
     expect(toolSet.definitions.map((tool) => tool.name)).toEqual([
       "direct_fail",
+      "direct_issues",
       "direct_lookup",
       "direct_media",
       "execute",
@@ -2033,6 +2074,39 @@ it.effect("returns content-only MCP results through Code Mode", () =>
   }),
 )
 
+it.effect("parses JSON text results from MCP tools without an output schema", () =>
+  Effect.gen(function* () {
+    assertion = yield* Deferred.make<Permission.AssertInput>()
+    decision = Effect.void
+    const registry = yield* Tool.Service
+    const registration = yield* McpTool.Service
+    yield* registration.flush
+    const toolSet = yield* registry.snapshot()
+
+    const run = (code: string) =>
+      toolSet
+        .execute({
+          sessionID: Session.ID.make("ses_mcp_json_text"),
+          ...toolIdentity,
+          call: { type: "tool-call", id: `call_${code.length}`, name: "execute", input: { code } },
+        })
+        .pipe(Effect.map((execution) => execution.output.output))
+
+    expect(yield* run("return (await tools.demo.issues({})).issues[0].id")).toBe("1")
+    expect(yield* run("return typeof (await tools.demo.count({}))")).toBe("string")
+    expect(yield* run("return typeof (await tools.demo.typed({}))")).toBe("string")
+
+    // Outside Code Mode the content the model reads is the original text.
+    expect(
+      yield* toolSet.execute({
+        sessionID: Session.ID.make("ses_mcp_json_text"),
+        ...toolIdentity,
+        call: { type: "tool-call", id: "call_direct_issues", name: "direct_issues", input: {} },
+      }),
+    ).toMatchObject({ output: { issues: [{ id: 1 }] }, content: [{ type: "text", text: '{"issues":[{"id":1}]}' }] })
+  }),
+)
+
 it.effect("advertises MCP tools directly when Code Mode is disabled for the server", () =>
   Effect.gen(function* () {
     const registry = yield* Tool.Service