Fix MCP 2026-07-28 listings failing for resources, templates and prompts registered as plain objects

.changeset/mcp-v2026-plain-descriptors.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix 2026-07-28 MCP listings failing for resources, resource templates and prompts registered as plain objects.

packages/effect/src/ai/internal/mcpProtocol/v2026_07_28.ts

@@ -203,13 +203,12 @@ const projectContent = Match.type<PublicMcpSchema.ContentBlock>().pipe(
   Match.exhaustive
 )
 
-const projectResource = (resource: PublicMcpSchema.Resource) =>
-  McpProtocol.transcode(PublicMcpSchema.Resource, McpSchema.Resource, resource)
+// Registrations may be plain objects, so encode with the wire struct.
+const projectResource = (resource: PublicMcpSchema.Resource) => Schema.encodeEffect(McpSchema.Resource)(resource)
 const projectResourceTemplate = (resourceTemplate: PublicMcpSchema.ResourceTemplate) =>
-  McpProtocol.transcode(PublicMcpSchema.ResourceTemplate, McpSchema.ResourceTemplate, resourceTemplate)
+  Schema.encodeEffect(McpSchema.ResourceTemplate)(resourceTemplate)
 
-const projectPrompt = (prompt: PublicMcpSchema.Prompt) =>
-  McpProtocol.transcode(PublicMcpSchema.Prompt, McpSchema.Prompt, prompt)
+const projectPrompt = (prompt: PublicMcpSchema.Prompt) => Schema.encodeEffect(McpSchema.Prompt)(prompt)
 
 const projectTool = (tool: PublicMcpSchema.Tool) => McpProtocol.transcode(PublicMcpSchema.Tool, McpSchema.Tool, tool)
 

packages/effect/test/ai/McpServer/v2026_07_28.test.ts

@@ -2,6 +2,7 @@ import { assert, describe, it } from "@effect/vitest"
 import * as McpProtocol from "effect/ai/McpProtocol"
 import * as McpSchema from "effect/ai/McpSchema"
 import * as McpServer from "effect/ai/McpServer"
+import * as Context from "effect/Context"
 import * as Duration from "effect/Duration"
 import * as Effect from "effect/Effect"
 import * as Base64 from "effect/encoding/Base64"
@@ -544,6 +545,53 @@ it.layer(testLayer)(`Mcp Conformance (${protocol.protocolVersion})`, (it) => {
         ])
       }))
 
+    it.effect("should list resources, templates and prompts registered as plain objects", () =>
+      Effect.gen(function*() {
+        const icon = { src: "https://example.com/icon.png" }
+        const registrations = Layer.effectDiscard(Effect.gen(function*() {
+          const server = yield* McpServer.McpServer
+          yield* server.addResource({
+            resource: { uri: "file:///plain", name: "Plain", icons: [icon] },
+            annotations: Context.empty(),
+            handle: Effect.succeed({ contents: [] })
+          })
+          yield* server.addResourceTemplate({
+            template: { uriTemplate: "file:///plain/{id}", name: "PlainTemplate", icons: [icon] },
+            annotations: Context.empty(),
+            routerPath: "file:///plain/:id",
+            completions: {},
+            handle: () => Effect.succeed({ contents: [] })
+          })
+          yield* server.addPrompt({
+            prompt: { name: "PlainPrompt", icons: [icon] },
+            annotations: Context.empty(),
+            completions: {},
+            handle: () => Effect.succeed({ messages: [] })
+          })
+        }))
+        const harness = yield* makeHttpHarness(registrations.pipe(Layer.provideMerge(makeServerLayer({
+          name: "PlainDescriptorsServer",
+          protocols: [protocol]
+        }))))
+        const list = Effect.fnUntraced(function*(method: string) {
+          const response = yield* harness.post(request(method, method), headers(method))
+          return Schema.decodeUnknownSync(Schema.Struct({ result: Schema.JsonObject }))(
+            yield* readMcpHttpResponse(response)
+          ).result
+        })
+
+        const resources = yield* list("resources/list")
+        assert.deepStrictEqual(resources.resources, [{ uri: "file:///plain", name: "Plain", icons: [icon] }])
+        const templates = yield* list("resources/templates/list")
+        assert.deepStrictEqual(templates.resourceTemplates, [{
+          uriTemplate: "file:///plain/{id}",
+          name: "PlainTemplate",
+          icons: [icon]
+        }])
+        const prompts = yield* list("prompts/list")
+        assert.deepStrictEqual(prompts.prompts, [{ name: "PlainPrompt", icons: [icon] }])
+      }))
+
     // SEP-2549 cache hints are server-advertised freshness; omitted families and resource reads keep the stale default.
     it.effect("should advertise configured cache hints and keep defaults for other results", () =>
       Effect.gen(function*() {