feat(opencode): support MCP resource content

packages/opencode/src/mcp/content.ts

@@ -0,0 +1,39 @@
+import { SessionV1 } from "@opencode-ai/core/v1/session"
+import { isImageAttachment } from "@/util/media"
+
+type ResourceContent = {
+  uri: string
+  mimeType?: string
+  text?: string
+  blob?: string
+}
+
+export type Part = SessionV1.TextPartInput | SessionV1.FilePartInput
+
+export function toParts(contents: readonly ResourceContent[]): Part[] {
+  return contents.flatMap((content) => {
+    if (content.text) return [{ type: "text", text: content.text }]
+    if (!content.blob) return []
+
+    const mime = content.mimeType?.split(";", 1)[0]?.trim().toLowerCase() || "application/octet-stream"
+    if (isImageAttachment(mime)) {
+      return [
+        {
+          type: "file",
+          mime,
+          filename: content.uri,
+          url: `data:${mime};base64,${content.blob}`,
+        },
+      ]
+    }
+
+    return [
+      {
+        type: "text",
+        text: `[Binary resource: ${mime}, ${Buffer.from(content.blob, "base64").byteLength} bytes]`,
+      },
+    ]
+  })
+}
+
+export * as McpContent from "./content"

packages/opencode/src/session/message-v2.ts

@@ -220,7 +220,12 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
             text: part.text,
           })
         // text/plain and directory files are converted into text parts, ignore them
-        if (part.type === "file" && part.mime !== "text/plain" && part.mime !== "application/x-directory") {
+        if (
+          part.type === "file" &&
+          part.mime !== "text/plain" &&
+          part.mime !== "application/x-directory" &&
+          part.source?.type !== "resource"
+        ) {
           if (options?.stripMedia && isMedia(part.mime)) {
             userMessage.parts.push({
               type: "text",

packages/opencode/src/session/prompt.ts

@@ -19,6 +19,7 @@ import { Plugin } from "../plugin"
 import MAX_STEPS from "../session/prompt/max-steps.txt"
 import { ToolRegistry } from "@/tool/registry"
 import { MCP } from "../mcp"
+import { McpContent } from "@/mcp/content"
 import { LSP } from "@/lsp/lsp"
 import { ulid } from "ulid"
 import { ChildProcess, ChildProcessSpawner } from "effect/unstable/process"
@@ -729,27 +730,14 @@ export const layer = Layer.effect(
             if (Exit.isSuccess(exit)) {
               const content = exit.value
               if (!content) throw new Error(`Resource not found: ${clientName}/${uri}`)
-              const items = Array.isArray(content.contents) ? content.contents : [content.contents]
-              for (const c of items) {
-                if ("text" in c && c.text) {
-                  pieces.push({
-                    messageID: info.id,
-                    sessionID: input.sessionID,
-                    type: "text",
-                    synthetic: true,
-                    text: c.text,
-                  })
-                } else if ("blob" in c && c.blob) {
-                  const mime = "mimeType" in c ? c.mimeType : part.mime
-                  pieces.push({
-                    messageID: info.id,
-                    sessionID: input.sessionID,
-                    type: "text",
-                    synthetic: true,
-                    text: `[Binary content: ${mime}]`,
-                  })
-                }
-              }
+              pieces.push(
+                ...McpContent.toParts(content.contents).map((item) => ({
+                  ...item,
+                  messageID: info.id,
+                  sessionID: input.sessionID,
+                  ...(item.type === "text" ? { synthetic: true } : {}),
+                })),
+              )
               pieces.push({ ...part, messageID: info.id, sessionID: input.sessionID })
             } else {
               const error = Cause.squash(exit.cause)
@@ -992,7 +980,7 @@ export const layer = Layer.effect(
       )
 
       const parts = yield* Effect.forEach(resolvedParts, (part) =>
-        part.type === "file" && part.mime.startsWith("image/")
+        part.type === "file" && part.source?.type !== "resource" && part.mime.startsWith("image/")
           ? image.normalize(part).pipe(
               Effect.catchIf(
                 (error) => error instanceof Image.ResizerUnavailableError,

packages/opencode/src/tool/mcp-resource.ts

@@ -0,0 +1,94 @@
+import { MCP } from "@/mcp"
+import { McpContent } from "@/mcp/content"
+import { Effect, Schema } from "effect"
+import * as Tool from "./tool"
+
+export const ListParameters = Schema.Struct({
+  clientName: Schema.optional(Schema.String).annotate({
+    description: "Only list resources from this MCP server",
+  }),
+})
+
+export const ReadParameters = Schema.Struct({
+  clientName: Schema.String.annotate({
+    description: "The MCP server that provides the resource",
+  }),
+  uri: Schema.String.annotate({
+    description: "The resource URI to read",
+  }),
+})
+
+export const ListMcpResourcesTool = Tool.define(
+  "list_mcp_resources",
+  Effect.gen(function* () {
+    const mcp = yield* MCP.Service
+
+    return {
+      description:
+        "List resources exposed by connected MCP servers. Resources are read-only context such as files, schemas, documents, images, and application data. Use read_mcp_resource with the returned clientName and uri to retrieve one.",
+      parameters: ListParameters,
+      execute: (params: Schema.Schema.Type<typeof ListParameters>) =>
+        Effect.gen(function* () {
+          const resources = Object.values(yield* mcp.resources())
+            .filter((resource) => !params.clientName || resource.client === params.clientName)
+            .toSorted(
+              (a, b) => a.client.localeCompare(b.client) || a.name.localeCompare(b.name) || a.uri.localeCompare(b.uri),
+            )
+            .map((resource) => ({
+              clientName: resource.client,
+              name: resource.name,
+              uri: resource.uri,
+              ...(resource.description ? { description: resource.description } : {}),
+              ...(resource.mimeType ? { mimeType: resource.mimeType } : {}),
+            }))
+
+          return {
+            title: "MCP resources",
+            metadata: { count: resources.length },
+            output: resources.length ? JSON.stringify({ resources }, null, 2) : "No MCP resources found.",
+          }
+        }),
+    }
+  }),
+)
+
+export const ReadMcpResourceTool = Tool.define(
+  "read_mcp_resource",
+  Effect.gen(function* () {
+    const mcp = yield* MCP.Service
+
+    return {
+      description:
+        "Read a resource from a connected MCP server. Use list_mcp_resources to discover resources when needed. Text is returned directly, images are attached for inspection, and other binary data is described by MIME type and size.",
+      parameters: ReadParameters,
+      execute: (params: Schema.Schema.Type<typeof ReadParameters>, ctx: Tool.Context) =>
+        Effect.gen(function* () {
+          yield* ctx.ask({
+            permission: "read_mcp_resource",
+            patterns: [`${params.clientName}:${params.uri}`],
+            always: [`${params.clientName}:${params.uri}`],
+            metadata: { clientName: params.clientName, uri: params.uri },
+          })
+
+          const result = yield* mcp.readResource(params.clientName, params.uri)
+          if (!result) throw new Error(`Failed to read MCP resource ${params.uri} from ${params.clientName}`)
+
+          const parts = McpContent.toParts(result.contents)
+          const output = parts
+            .filter((part): part is Extract<McpContent.Part, { type: "text" }> => part.type === "text")
+            .map((part) => part.text)
+            .join("\n\n")
+          const attachments = parts.filter(
+            (part): part is Extract<McpContent.Part, { type: "file" }> => part.type === "file",
+          )
+
+          return {
+            title: `${params.clientName}: ${params.uri}`,
+            metadata: { clientName: params.clientName, uri: params.uri },
+            output: output || `Read MCP resource ${params.uri}`,
+            ...(attachments.length ? { attachments } : {}),
+          }
+        }),
+    }
+  }),
+)

packages/opencode/src/tool/registry.ts

@@ -52,6 +52,8 @@ import { BackgroundJob } from "@/background/job"
 import { RuntimeFlags } from "@/effect/runtime-flags"
 import { ProviderV2 } from "@opencode-ai/core/provider"
 import { ModelV2 } from "@opencode-ai/core/model"
+import { ListMcpResourcesTool, ReadMcpResourceTool } from "./mcp-resource"
+import { MCP } from "@/mcp"
 
 export function webSearchEnabled(providerID: ProviderV2.ID, flags = { exa: false, parallel: false }) {
   return providerID === ProviderV2.ID.opencode || flags.exa || flags.parallel
@@ -105,6 +107,8 @@ export const layer = Layer.effect(
     const greptool = yield* GrepTool
     const patchtool = yield* ApplyPatchTool
     const skilltool = yield* SkillTool
+    const listMcpResources = yield* ListMcpResourcesTool
+    const readMcpResource = yield* ReadMcpResourceTool
     const agent = yield* Agent.Service
 
     const state = yield* InstanceState.make<State>(
@@ -212,6 +216,8 @@ export const layer = Layer.effect(
           question: Tool.init(question),
           lsp: Tool.init(lsptool),
           plan: Tool.init(plan),
+          listMcpResources: Tool.init(listMcpResources),
+          readMcpResource: Tool.init(readMcpResource),
         })
 
         return {
@@ -231,6 +237,8 @@ export const layer = Layer.effect(
             tool.search,
             tool.skill,
             tool.patch,
+            tool.listMcpResources,
+            tool.readMcpResource,
             ...(flags.experimentalLspTool ? [tool.lsp] : []),
             ...(flags.experimentalPlanMode && flags.client === "cli" ? [tool.plan] : []),
           ],
@@ -335,6 +343,7 @@ export const defaultLayer = Layer.suspend(() =>
       Layer.provide(Format.defaultLayer),
       Layer.provide(CrossSpawnSpawner.defaultLayer),
       Layer.provide(Truncate.defaultLayer),
+      Layer.provide(MCP.defaultLayer),
     )
     .pipe(Layer.provide(Database.defaultLayer), Layer.provide(RuntimeFlags.defaultLayer)),
 )
@@ -435,6 +444,7 @@ export const node = LayerNode.make(layer.pipe(Layer.provide(Ripgrep.defaultLayer
   Truncate.node,
   RuntimeFlags.node,
   Database.node,
+  MCP.node,
 ])
 
 export * as ToolRegistry from "./registry"

packages/opencode/test/mcp/content.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, test } from "bun:test"
+import { McpContent } from "@/mcp/content"
+
+describe("mcp.content", () => {
+  test("converts text, images, and other binary resources", () => {
+    expect(
+      McpContent.toParts([
+        { uri: "fixture://guide", mimeType: "text/plain", text: "hello" },
+        { uri: "fixture://picture", mimeType: "IMAGE/PNG; charset=binary", blob: "aGVsbG8=" },
+        { uri: "fixture://archive", mimeType: "application/zip", blob: "aGVsbG8=" },
+      ]),
+    ).toEqual([
+      { type: "text", text: "hello" },
+      {
+        type: "file",
+        mime: "image/png",
+        filename: "fixture://picture",
+        url: "data:image/png;base64,aGVsbG8=",
+      },
+      { type: "text", text: "[Binary resource: application/zip, 5 bytes]" },
+    ])
+  })
+
+  test("does not treat SVG resources as model images", () => {
+    expect(McpContent.toParts([{ uri: "fixture://vector", mimeType: "image/svg+xml", blob: "PHN2Zy8+" }])).toEqual([
+      { type: "text", text: "[Binary resource: image/svg+xml, 6 bytes]" },
+    ])
+  })
+})

packages/opencode/test/session/message-v2.test.ts

@@ -319,6 +319,43 @@ describe("session.message-v2.toModelMessage", () => {
     ])
   })
 
+  test("does not forward MCP resource references as file downloads", async () => {
+    const messageID = "m-user"
+    const input: SessionV1.WithParts[] = [
+      {
+        info: userInfo(messageID),
+        parts: [
+          {
+            ...basePart(messageID, "p1"),
+            type: "text",
+            text: "# Resource contents",
+            synthetic: true,
+          },
+          {
+            ...basePart(messageID, "p2"),
+            type: "file",
+            mime: "application/json",
+            filename: "status",
+            url: "status://info",
+            source: {
+              type: "resource",
+              clientName: "resource-only-fixture",
+              uri: "status://info",
+              text: { value: "@status", start: 0, end: 7 },
+            },
+          },
+        ] as SessionV1.Part[],
+      },
+    ]
+
+    expect(await MessageV2.toModelMessages(input, model)).toStrictEqual([
+      {
+        role: "user",
+        content: [{ type: "text", text: "# Resource contents" }],
+      },
+    ])
+  })
+
   test("converts assistant tool completion into tool-call + tool-result messages with attachments", async () => {
     const userID = "m-user"
     const assistantID = "m-assistant"

packages/opencode/test/tool/mcp-resource.test.ts

@@ -0,0 +1,107 @@
+import { PermissionV1 } from "@opencode-ai/core/v1/permission"
+import { describe, expect } from "bun:test"
+import { Effect, Layer } from "effect"
+import { Agent } from "@/agent/agent"
+import { MCP } from "@/mcp"
+import { MessageID, SessionID } from "@/session/schema"
+import { ListMcpResourcesTool, ReadMcpResourceTool } from "@/tool/mcp-resource"
+import { Tool } from "@/tool/tool"
+import { Truncate } from "@/tool/truncate"
+import { testEffect } from "../lib/effect"
+
+const mcp = Layer.mock(MCP.Service, {
+  resources: () =>
+    Effect.succeed({
+      "zeta:guide": {
+        client: "zeta",
+        name: "guide",
+        uri: "fixture://guide",
+        description: "Guide",
+        mimeType: "text/plain",
+      },
+      "alpha:picture": {
+        client: "alpha",
+        name: "picture",
+        uri: "fixture://picture",
+        mimeType: "image/png",
+      },
+    }),
+  readResource: (clientName, uri) =>
+    Effect.succeed(
+      clientName === "alpha" && uri === "fixture://picture"
+        ? {
+            contents: [
+              { uri, mimeType: "text/plain", text: "caption" },
+              { uri, mimeType: "image/png", blob: "aGVsbG8=" },
+            ],
+          }
+        : undefined,
+    ),
+})
+
+const it = testEffect(Layer.mergeAll(mcp, Truncate.defaultLayer, Agent.defaultLayer))
+
+const baseCtx: Omit<Tool.Context, "ask"> = {
+  sessionID: SessionID.make("ses_test"),
+  messageID: MessageID.make("msg_test"),
+  agent: "build",
+  abort: AbortSignal.any([]),
+  messages: [],
+  metadata: () => Effect.void,
+}
+
+describe("tool.mcp-resource", () => {
+  it.instance("lists resources in a deterministic order", () =>
+    Effect.gen(function* () {
+      const info = yield* ListMcpResourcesTool
+      const tool = yield* info.init()
+      const result = yield* tool.execute({}, { ...baseCtx, ask: () => Effect.void })
+
+      expect(JSON.parse(result.output)).toEqual({
+        resources: [
+          { clientName: "alpha", name: "picture", uri: "fixture://picture", mimeType: "image/png" },
+          {
+            clientName: "zeta",
+            name: "guide",
+            uri: "fixture://guide",
+            description: "Guide",
+            mimeType: "text/plain",
+          },
+        ],
+      })
+    }),
+  )
+
+  it.instance("returns text and image content from a resource", () =>
+    Effect.gen(function* () {
+      const requests: Array<Omit<PermissionV1.Request, "id" | "sessionID" | "tool">> = []
+      const info = yield* ReadMcpResourceTool
+      const tool = yield* info.init()
+      const result = yield* tool.execute(
+        { clientName: "alpha", uri: "fixture://picture" },
+        {
+          ...baseCtx,
+          ask: (request) =>
+            Effect.sync(() => {
+              requests.push(request)
+            }),
+        },
+      )
+
+      expect(result.output).toBe("caption")
+      expect(result.attachments).toEqual([
+        {
+          type: "file",
+          mime: "image/png",
+          filename: "fixture://picture",
+          url: "data:image/png;base64,aGVsbG8=",
+        },
+      ])
+      expect(requests[0]).toMatchObject({
+        permission: "read_mcp_resource",
+        patterns: ["alpha:fixture://picture"],
+        always: ["alpha:fixture://picture"],
+      })
+    }),
+  )
+})

packages/opencode/test/tool/registry.test.ts

@@ -72,6 +72,8 @@ describe("tool.registry", () => {
       const ids = yield* registry.ids()
 
       expect(ids).not.toContain("task_status")
+      expect(ids).toContain("list_mcp_resources")
+      expect(ids).toContain("read_mcp_resource")
     }),
   )