feat(mcp): append server instructions to context (#32490)

Co-authored-by: Aiden Cline <aidenpcline@gmail.com>

packages/opencode/src/mcp/catalog.ts

@@ -115,6 +115,8 @@ export function fetch<T extends { name: string }>(
 
 export const sanitize = (value: string) => value.replace(/[^a-zA-Z0-9_-]/g, "_")
 
+export const toolName = (clientName: string, name: string) => sanitize(clientName) + "_" + sanitize(name)
+
 export function prompts(client: Client, timeout?: number) {
   if (!client.getServerCapabilities()?.prompts) return Promise.resolve([])
   return paginate(

packages/opencode/src/mcp/index.ts

@@ -140,6 +140,7 @@ interface CreateResult {
   mcpClient?: MCPClient
   status: Status
   defs?: MCPToolDef[]
+  instructions?: string
 }
 
 interface AuthResult {
@@ -155,11 +156,19 @@ interface State {
   status: Record<string, Status>
   clients: Record<string, MCPClient>
   defs: Record<string, MCPToolDef[]>
+  instructions: Record<string, string>
+}
+
+export interface ServerInstructions {
+  name: string
+  instructions: string
+  tools: string[]
 }
 
 export interface Interface {
   readonly status: () => Effect.Effect<Record<string, Status>>
   readonly clients: () => Effect.Effect<Record<string, MCPClient>>
+  readonly instructions: () => Effect.Effect<ServerInstructions[]>
   readonly tools: () => Effect.Effect<Record<string, Tool>>
   readonly prompts: () => Effect.Effect<Record<string, PromptInfo & { client: string }>>
   readonly resources: (clientName?: string) => Effect.Effect<Record<string, ResourceInfo & { client: string }>>
@@ -383,7 +392,7 @@ export const layer = Layer.effect(
           if (!listed) {
             return yield* Effect.fail(new Error("Failed to get tools"))
           }
-          return { mcpClient, status, defs: listed } satisfies CreateResult
+          return { mcpClient, status, defs: listed, instructions: mcpClient.getInstructions()?.trim() } satisfies CreateResult
         }).pipe(
           Effect.catchCause((cause) =>
             Effect.tryPromise(() => mcpClient.close()).pipe(Effect.ignore, Effect.andThen(Effect.failCause(cause))),
@@ -430,6 +439,7 @@ export const layer = Layer.effect(
         if (s.clients[name] !== client) return
         delete s.clients[name]
         delete s.defs[name]
+        delete s.instructions[name]
         s.status[name] = { status: "failed", error: "Connection closed" }
         bridge.fork(
           Effect.logWarning("MCP connection closed", { server: name }).pipe(
@@ -484,6 +494,7 @@ export const layer = Layer.effect(
           status: {},
           clients: {},
           defs: {},
+          instructions: {},
         }
 
         yield* Effect.forEach(
@@ -505,6 +516,7 @@ export const layer = Layer.effect(
               if (result.mcpClient) {
                 s.clients[key] = result.mcpClient
                 s.defs[key] = result.defs!
+                if (result.instructions) s.instructions[key] = result.instructions
                 watch(s, key, result.mcpClient, bridge, mcp.timeout)
               }
             }),
@@ -516,6 +528,7 @@ export const layer = Layer.effect(
             const clients = Object.values(s.clients)
             s.clients = {}
             s.defs = {}
+            s.instructions = {}
             yield* Effect.forEach(
               clients,
               (client) =>
@@ -545,6 +558,7 @@ export const layer = Layer.effect(
       const client = s.clients[name]
       delete s.clients[name]
       delete s.defs[name]
+      delete s.instructions[name]
       if (!client) return Effect.void
       return Effect.tryPromise(() => client.close()).pipe(Effect.ignore)
     }
@@ -554,6 +568,7 @@ export const layer = Layer.effect(
       name: string,
       client: MCPClient,
       listed: MCPToolDef[],
+      instructions: string | undefined,
       timeout?: number,
     ) {
       const bridge = yield* EffectBridge.make()
@@ -561,6 +576,8 @@ export const layer = Layer.effect(
       s.status[name] = { status: "connected" }
       s.clients[name] = client
       s.defs[name] = listed
+      if (instructions) s.instructions[name] = instructions
+      else delete s.instructions[name]
       watch(s, name, client, bridge, timeout)
       if (previous) yield* Effect.tryPromise(() => previous.close()).pipe(Effect.ignore)
       return s.status[name]
@@ -590,6 +607,18 @@ export const layer = Layer.effect(
       return s.clients
     })
 
+    const instructions = Effect.fn("MCP.instructions")(function* () {
+      const s = yield* InstanceState.get(state)
+      return Object.entries(s.instructions)
+        .filter(([name]) => s.status[name]?.status === "connected")
+        .sort(([a], [b]) => a.localeCompare(b))
+        .map(([name, item]) => ({
+          name,
+          instructions: item,
+          tools: (s.defs[name] ?? []).map((tool) => McpCatalog.toolName(name, tool.name)),
+        }))
+    })
+
     const createAndStore = Effect.fn("MCP.createAndStore")(function* (name: string, mcp: ConfigMCPV1.Info) {
       const s = yield* InstanceState.get(state)
       const result = yield* create(name, mcp)
@@ -601,7 +630,7 @@ export const layer = Layer.effect(
         return result.status
       }
 
-      return yield* storeClient(s, name, result.mcpClient, result.defs!, mcp.timeout)
+      return yield* storeClient(s, name, result.mcpClient, result.defs!, result.instructions, mcp.timeout)
     })
 
     const add = Effect.fn("MCP.add")(function* (name: string, mcp: ConfigMCPV1.Info) {
@@ -647,7 +676,7 @@ export const layer = Layer.effect(
         }
         const timeout = requestTimeout(s, clientName, mcpConfig, defaultTimeout)
         for (const mcpTool of listed) {
-          const key = McpCatalog.sanitize(clientName) + "_" + McpCatalog.sanitize(mcpTool.name)
+          const key = McpCatalog.toolName(clientName, mcpTool.name)
           result[key] = McpCatalog.convertTool(mcpTool, client, timeout)
         }
       }
@@ -855,7 +884,7 @@ export const layer = Layer.effect(
 
         const s = yield* InstanceState.get(state)
         yield* auth.clearOAuthState(mcpName)
-        return yield* storeClient(s, mcpName, client, listed, mcpConfig.timeout)
+        return yield* storeClient(s, mcpName, client, listed, client.getInstructions()?.trim(), mcpConfig.timeout)
       }
 
       const callbackPromise = McpOAuthCallback.waitForCallback(result.oauthState, mcpName)
@@ -942,6 +971,7 @@ export const layer = Layer.effect(
     return Service.of({
       status,
       clients,
+      instructions,
       tools,
       prompts,
       resources,

packages/opencode/src/session/prompt.ts

@@ -1356,13 +1356,19 @@ export const layer = Layer.effect(
 
             yield* plugin.trigger("experimental.chat.messages.transform", {}, { messages: msgs })
 
-            const [skills, env, instructions, modelMsgs] = yield* Effect.all([
+            const [skills, env, instructions, mcpInstructions, modelMsgs] = yield* Effect.all([
               sys.skills(agent),
               sys.environment(model),
               instruction.system().pipe(Effect.orDie),
+              sys.mcp(agent, session.permission),
               MessageV2.toModelMessagesEffect(msgs, model),
             ])
-            const system = [...env, ...instructions, ...(skills ? [skills] : [])]
+            const system = [
+              ...env,
+              ...instructions,
+              ...(mcpInstructions ? [mcpInstructions] : []),
+              ...(skills ? [skills] : []),
+            ]
             const format = lastUser.format ?? { type: "text" as const }
             if (format.type === "json_schema") system.push(STRUCTURED_OUTPUT_SYSTEM_PROMPT)
             const result = yield* handle.process({

packages/opencode/src/session/system.ts

@@ -20,6 +20,8 @@ import { AbsolutePath } from "@opencode-ai/core/schema"
 import { Location } from "@opencode-ai/core/location"
 import { LocationServiceMap } from "@opencode-ai/core/location-layer"
 import { Reference } from "@opencode-ai/core/reference"
+import { MCP } from "@/mcp"
+import { PermissionV1 } from "@opencode-ai/core/v1/permission"
 
 export function provider(model: Provider.Model) {
   if (model.api.id.includes("gpt-4") || model.api.id.includes("o1") || model.api.id.includes("o3"))
@@ -40,6 +42,7 @@ export function provider(model: Provider.Model) {
 export interface Interface {
   readonly environment: (model: Provider.Model) => Effect.Effect<string[]>
   readonly skills: (agent: Agent.Info) => Effect.Effect<string | undefined>
+  readonly mcp: (agent: Agent.Info, permission?: PermissionV1.Ruleset) => Effect.Effect<string | undefined>
 }
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/SystemPrompt") {}
@@ -48,6 +51,7 @@ export const layer = Layer.effect(
   Service,
   Effect.gen(function* () {
     const skill = yield* Skill.Service
+    const mcp = yield* MCP.Service
     const locations = yield* LocationServiceMap
 
     return Service.of({
@@ -102,14 +106,36 @@ export const layer = Layer.effect(
           Skill.fmt(list, { verbose: true }),
         ].join("\n")
       }),
+
+      mcp: Effect.fn("SystemPrompt.mcp")(function* (agent: Agent.Info, permission?: PermissionV1.Ruleset) {
+        const ruleset = Permission.merge(agent.permission, permission ?? [])
+        const instructions = (yield* mcp.instructions()).filter(
+          (item) => item.tools.length === 0 || Permission.disabled(item.tools, ruleset).size < item.tools.length,
+        )
+        if (instructions.length === 0) return
+
+        return [
+          "<mcp_instructions>",
+          ...instructions.flatMap((item) => [
+            `  <server name="${item.name}">`,
+            ...item.instructions.split("\n").map((line) => `    ${line}`),
+            "  </server>",
+          ]),
+          "</mcp_instructions>",
+        ].join("\n")
+      }),
     })
   }),
 )
 
-export const defaultLayer = layer.pipe(Layer.provide(Skill.defaultLayer), Layer.provide(LocationServiceMap.layer))
+export const defaultLayer = layer.pipe(
+  Layer.provide(Skill.defaultLayer),
+  Layer.provide(MCP.defaultLayer),
+  Layer.provide(LocationServiceMap.layer),
+)
 
 const locationServiceMapNode = LayerNode.make(LocationServiceMap.layer, [])
 
-export const node = LayerNode.make(layer, [Skill.node, locationServiceMapNode])
+export const node = LayerNode.make(layer, [Skill.node, MCP.node, locationServiceMapNode])
 
 export * as SystemPrompt from "./system"

packages/opencode/test/mcp/lifecycle.test.ts

@@ -13,6 +13,7 @@ import { TestInstance } from "../fixture/fixture"
 interface MockClientState {
   capabilities: { tools?: object; prompts?: object; resources?: object }
   capabilitiesShouldThrow: boolean
+  instructions?: string
   tools: Array<{ name: string; description?: string; inputSchema: object; outputSchema?: object }>
   listToolsCalls: number
   listPromptsCalls: number
@@ -188,6 +189,10 @@ void mock.module("@modelcontextprotocol/sdk/client/index.js", () => ({
       return this._state?.capabilities
     }
 
+    getInstructions() {
+      return this._state?.instructions
+    }
+
     async listTools(params?: { cursor?: string }) {
       if (this._state) this._state.listToolsCalls++
       if (this._state?.listToolsShouldFail) {
@@ -347,6 +352,60 @@ it.instance(
   { config: { mcp: {} } },
 )
 
+it.instance(
+  "instructions() returns connected server instructions with tool names",
+  () =>
+    MCP.Service.use((mcp: MCPNS.Interface) =>
+      Effect.gen(function* () {
+        lastCreatedClientName = "guide-server"
+        const serverState = getOrCreateClientState("guide-server")
+        serverState.instructions = "Use lookup before mutate."
+
+        yield* mcp.add("guide-server", {
+          type: "local",
+          command: ["echo", "test"],
+        })
+
+        expect(yield* mcp.instructions()).toContainEqual({
+          name: "guide-server",
+          instructions: "Use lookup before mutate.",
+          tools: ["guide-server_test_tool"],
+        })
+      }),
+    ),
+  { config: { mcp: {} } },
+)
+
+it.instance(
+  "instructions() omits empty and disconnected server instructions",
+  () =>
+    MCP.Service.use((mcp: MCPNS.Interface) =>
+      Effect.gen(function* () {
+        lastCreatedClientName = "temporary-server"
+        getOrCreateClientState("temporary-server").instructions = "Temporary guidance."
+
+        yield* mcp.add("temporary-server", {
+          type: "local",
+          command: ["echo", "test"],
+        })
+        yield* mcp.disconnect("temporary-server")
+
+        lastCreatedClientName = "blank-server"
+        getOrCreateClientState("blank-server").instructions = "   "
+
+        yield* mcp.add("blank-server", {
+          type: "local",
+          command: ["echo", "test"],
+        })
+
+        const instructions = yield* mcp.instructions()
+        expect(instructions.some((item) => item.name === "temporary-server")).toBe(false)
+        expect(instructions.some((item) => item.name === "blank-server")).toBe(false)
+      }),
+    ),
+  { config: { mcp: {} } },
+)
+
 it.instance(
   "follows cursors when listing tools, prompts, and resources",
   () =>

packages/opencode/test/session/prompt.test.ts

@@ -56,6 +56,7 @@ import { reply, TestLLMServer } from "../lib/llm-server"
 import { RuntimeFlags } from "@/effect/runtime-flags"
 import { ProviderV2 } from "@opencode-ai/core/provider"
 import { ModelV2 } from "@opencode-ai/core/model"
+import { LocationServiceMap } from "@opencode-ai/core/location-layer"
 
 const summary = Layer.succeed(
   SessionSummary.Service,
@@ -108,29 +109,32 @@ function errorTool(parts: SessionV1.Part[]) {
   return part?.state.status === "error" ? (part as ErrorToolPart) : undefined
 }
 
-const mcp = Layer.succeed(
-  MCP.Service,
-  MCP.Service.of({
-    status: () => Effect.succeed({}),
-    clients: () => Effect.succeed({}),
-    tools: () => Effect.succeed({}),
-    prompts: () => Effect.succeed({}),
-    resources: () => Effect.succeed({}),
-    resourceTemplates: () => Effect.succeed({}),
-    add: () => Effect.succeed({ status: { status: "disabled" as const } }),
-    connect: () => Effect.void,
-    disconnect: () => Effect.void,
-    getPrompt: () => Effect.succeed(undefined),
-    readResource: () => Effect.succeed(undefined),
-    startAuth: () => Effect.die("unexpected MCP auth in prompt-effect tests"),
-    authenticate: () => Effect.die("unexpected MCP auth in prompt-effect tests"),
-    finishAuth: () => Effect.die("unexpected MCP auth in prompt-effect tests"),
-    removeAuth: () => Effect.void,
-    supportsOAuth: () => Effect.succeed(false),
-    hasStoredTokens: () => Effect.succeed(false),
-    getAuthStatus: () => Effect.succeed("not_authenticated" as const),
-  }),
-)
+function makeMcp(instructions: MCP.ServerInstructions[] = []) {
+  return Layer.succeed(
+    MCP.Service,
+    MCP.Service.of({
+      status: () => Effect.succeed({}),
+      clients: () => Effect.succeed({}),
+      instructions: () => Effect.succeed(instructions),
+      tools: () => Effect.succeed({}),
+      prompts: () => Effect.succeed({}),
+      resources: () => Effect.succeed({}),
+      resourceTemplates: () => Effect.succeed({}),
+      add: () => Effect.succeed({ status: { status: "disabled" as const } }),
+      connect: () => Effect.void,
+      disconnect: () => Effect.void,
+      getPrompt: () => Effect.succeed(undefined),
+      readResource: () => Effect.succeed(undefined),
+      startAuth: () => Effect.die("unexpected MCP auth in prompt-effect tests"),
+      authenticate: () => Effect.die("unexpected MCP auth in prompt-effect tests"),
+      finishAuth: () => Effect.die("unexpected MCP auth in prompt-effect tests"),
+      removeAuth: () => Effect.void,
+      supportsOAuth: () => Effect.succeed(false),
+      hasStoredTokens: () => Effect.succeed(false),
+      getAuthStatus: () => Effect.succeed("not_authenticated" as const),
+    }),
+  )
+}
 
 const lsp = Layer.succeed(
   LSP.Service,
@@ -164,7 +168,10 @@ const blockingProcessor = Layer.succeed(
   }),
 )
 
-function makePrompt(input?: { processor?: "blocking" }) {
+function makePrompt(input?: {
+  mcpInstructions?: MCP.ServerInstructions[]
+  processor?: "blocking"
+}) {
   const deps = Layer.mergeAll(
     Session.defaultLayer,
     Snapshot.defaultLayer,
@@ -177,7 +184,7 @@ function makePrompt(input?: { processor?: "blocking" }) {
     Config.defaultLayer,
     ProviderSvc.defaultLayer,
     lsp,
-    mcp,
+    makeMcp(input?.mcpInstructions),
     FSUtil.defaultLayer,
     BackgroundJob.defaultLayer,
     status,
@@ -223,24 +230,47 @@ function makePrompt(input?: { processor?: "blocking" }) {
     Layer.provideMerge(registry),
     Layer.provideMerge(trunc),
     Layer.provide(Instruction.defaultLayer),
-    Layer.provide(SystemPrompt.defaultLayer),
+    Layer.provide(
+      SystemPrompt.layer.pipe(
+        Layer.provide(Skill.defaultLayer),
+        Layer.provide(LocationServiceMap.layer),
+        Layer.provide(deps),
+      ),
+    ),
     Layer.provide(RuntimeFlags.layer({ experimentalEventSystem: true })),
     Layer.provideMerge(deps),
     Layer.provide(summary),
   )
 }
 
-function makeHttp(input?: { processor?: "blocking" }) {
+function makeHttp(input?: {
+  mcpInstructions?: MCP.ServerInstructions[]
+  processor?: "blocking"
+}) {
   return Layer.mergeAll(TestLLMServer.layer, makePrompt(input))
 }
 
-function makeHttpNoLLMServer(input?: { processor?: "blocking" }) {
+function makeHttpNoLLMServer(input?: {
+  mcpInstructions?: MCP.ServerInstructions[]
+  processor?: "blocking"
+}) {
   return makePrompt(input)
 }
 
 const it = testEffect(makeHttp())
 const noLLMServer = testEffect(makeHttpNoLLMServer())
 const raceNoLLMServer = testEffect(makeHttpNoLLMServer({ processor: "blocking" }))
+const withMcpInstructions = testEffect(
+  makeHttp({
+    mcpInstructions: [
+      {
+        name: "guide-server",
+        instructions: "Use lookup before mutate.",
+        tools: ["guide-server_lookup"],
+      },
+    ],
+  }),
+)
 const unix = process.platform !== "win32" ? it.instance : it.instance.skip
 const unixNoLLMServer = process.platform !== "win32" ? noLLMServer.instance : noLLMServer.instance.skip
 
@@ -507,6 +537,30 @@ it.instance("loop calls LLM and returns assistant message", () =>
   }),
 )
 
+withMcpInstructions.instance("loop includes MCP instructions in model system context", () =>
+  Effect.gen(function* () {
+    const { llm } = yield* useServerConfig(providerCfg)
+    const prompt = yield* SessionPrompt.Service
+    const sessions = yield* Session.Service
+    const chat = yield* sessions.create({
+      title: "Pinned",
+      permission: [{ permission: "*", pattern: "*", action: "allow" }],
+    })
+    yield* llm.hang
+    yield* user(chat.id, "hello")
+
+    const fiber = yield* prompt.loop({ sessionID: chat.id }).pipe(Effect.forkChild)
+    yield* awaitWithTimeout(llm.wait(1), "timed out waiting for MCP instruction request", "10 seconds")
+
+    const hits = yield* llm.hits
+    const body = JSON.stringify(hits[0]?.body)
+    expect(body).toContain('<server name=\\"guide-server\\">')
+    expect(body).toContain("Use lookup before mutate.")
+    yield* Fiber.interrupt(fiber)
+  }),
+  15_000,
+)
+
 it.instance("loop surfaces content-filter finishes as session errors", () =>
   Effect.gen(function* () {
     const { llm } = yield* useServerConfig(providerCfg)

packages/opencode/test/session/snapshot-tool-race.test.ts

@@ -37,6 +37,7 @@ const mcp = Layer.succeed(
   MCP.Service.of({
     status: () => Effect.succeed({}),
     clients: () => Effect.succeed({}),
+    instructions: () => Effect.succeed([]),
     tools: () => Effect.succeed({}),
     prompts: () => Effect.succeed({}),
     resources: () => Effect.succeed({}),

packages/opencode/test/session/system.test.ts

@@ -5,6 +5,7 @@ import { NamedError } from "@opencode-ai/core/util/error"
 import { Skill } from "../../src/skill"
 import { Permission } from "../../src/permission"
 import { SystemPrompt } from "../../src/session/system"
+import { MCP } from "../../src/mcp"
 import { LocationServiceMap } from "@opencode-ai/core/location-layer"
 import { testEffect } from "../lib/effect"
 
@@ -44,6 +45,23 @@ const build: Agent.Info = {
 const it = testEffect(
   SystemPrompt.layer.pipe(
     Layer.provide(LocationServiceMap.layer),
+    Layer.provide(
+      Layer.mock(MCP.Service, {
+        instructions: () =>
+          Effect.succeed([
+            {
+              name: "guide-server",
+              instructions: "Use lookup before mutate.",
+              tools: [],
+            },
+            {
+              name: "tool-server",
+              instructions: "Prefer search before update.",
+              tools: ["tool-server_search", "tool-server_update"],
+            },
+          ]),
+      }),
+    ),
     Layer.provide(
       Layer.succeed(
         Skill.Service,
@@ -83,4 +101,41 @@ describe("session.system", () => {
       expect(output).not.toContain("manual-skill")
     }),
   )
+
+  it.effect("MCP output includes connected server instructions", () =>
+    Effect.gen(function* () {
+      const prompt = yield* SystemPrompt.Service
+      const output = yield* prompt.mcp(build)
+
+      expect(output).toBe(
+        [
+          "<mcp_instructions>",
+          '  <server name="guide-server">',
+          "    Use lookup before mutate.",
+          "  </server>",
+          '  <server name="tool-server">',
+          "    Prefer search before update.",
+          "  </server>",
+          "</mcp_instructions>",
+        ].join("\n"),
+      )
+    }),
+  )
+
+  it.effect("MCP output omits servers when all advertised tools are denied", () =>
+    Effect.gen(function* () {
+      const prompt = yield* SystemPrompt.Service
+      const output = yield* prompt.mcp(build, Permission.fromConfig({ "tool-server_*": "deny" }))
+
+      expect(output).toBe(
+        [
+          "<mcp_instructions>",
+          '  <server name="guide-server">',
+          "    Use lookup before mutate.",
+          "  </server>",
+          "</mcp_instructions>",
+        ].join("\n"),
+      )
+    }),
+  )
 })