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

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

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"