diff --git a/packages/opencode/src/cli/cmd/mcp.ts b/packages/opencode/src/cli/cmd/mcp.ts
index c9f74e2bc0..ba51259e26 100644
--- a/packages/opencode/src/cli/cmd/mcp.ts
+++ b/packages/opencode/src/cli/cmd/mcp.ts
@@ -5,7 +5,7 @@ import { Cause } from "effect"
 import { Client, StreamableHTTPClientTransport, UnauthorizedError } from "@modelcontextprotocol/client"
 import * as prompts from "@clack/prompts"
 import { UI } from "../ui"
-import { CLIENT_OPTIONS, MCP } from "../../mcp"
+import { clientOptions, MCP } from "../../mcp"
 import { McpAuth } from "../../mcp/auth"
 import { McpOAuthProvider } from "../../mcp/oauth-provider"
 import { Config } from "@/config/config"
@@ -751,7 +751,7 @@ export const McpDebugCommand = effectCmd({
         authProvider,
         requestInit: serverConfig.headers ? { headers: serverConfig.headers } : undefined,
       })
-      const client = new Client({ name: "opencode-debug", version: InstallationVersion }, CLIENT_OPTIONS)
+      const client = new Client({ name: "opencode-debug", version: InstallationVersion }, clientOptions())
 
       try {
         await client.connect(transport)
diff --git a/packages/opencode/src/mcp/index.ts b/packages/opencode/src/mcp/index.ts
index 939c442047..ec7be68b40 100644
--- a/packages/opencode/src/mcp/index.ts
+++ b/packages/opencode/src/mcp/index.ts
@@ -15,6 +15,7 @@ import {
   type Tool as MCPToolDef,
 } from "@modelcontextprotocol/client"
 import { StdioClientTransport } from "@modelcontextprotocol/client/stdio"
+import { Ajv, AjvJsonSchemaValidator, addFormats } from "@modelcontextprotocol/client/validators/ajv"
 import { Config } from "@/config/config"
 import { ConfigMCPV1 } from "@opencode-ai/core/v1/config/mcp"
 import { NamedError } from "@opencode-ai/core/util/error"
@@ -34,22 +35,39 @@ import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
 import { McpCatalog } from "./catalog"
 import { McpEvent } from "@opencode-ai/schema/mcp-event"
 import { McpBrowser } from "./browser"
+import { lazy } from "@/util/lazy"
 
 const DEFAULT_TIMEOUT = 30_000
-export const CLIENT_OPTIONS = {
-  capabilities: {
-    // https://github.com/anomalyco/opencode/issues/11948
-    // sampling: {},
-    // https://github.com/anomalyco/opencode/issues/23066
-    // elicitation: {},
-    // https://github.com/anomalyco/opencode/issues/2308
-    roots: {},
-    // https://github.com/anomalyco/opencode/issues/28567
-    // tasks: {},
-  },
-  versionNegotiation: { mode: "auto" },
-  listMaxPages: 1_000,
-} satisfies ClientOptions
+
+export function clientOptions(): ClientOptions {
+  const draft7Validator = lazy(() => {
+    const ajv = new Ajv({ strict: false, validateFormats: true, validateSchema: false, allErrors: true })
+    addFormats(ajv)
+    return new AjvJsonSchemaValidator(ajv)
+  })
+  const defaultValidator = new AjvJsonSchemaValidator()
+
+  return {
+    capabilities: {
+      // https://github.com/anomalyco/opencode/issues/11948
+      // sampling: {},
+      // https://github.com/anomalyco/opencode/issues/23066
+      // elicitation: {},
+      // https://github.com/anomalyco/opencode/issues/2308
+      roots: {},
+      // https://github.com/anomalyco/opencode/issues/28567
+      // tasks: {},
+    },
+    versionNegotiation: { mode: "auto" },
+    listMaxPages: 1_000,
+    jsonSchemaValidator: {
+      getValidator: <T>(schema: { $schema?: string }) => {
+        if (!schema.$schema?.toLowerCase().includes("draft-07")) return defaultValidator.getValidator<T>(schema)
+        return draft7Validator().getValidator<T>(schema)
+      },
+    },
+  }
+}
 
 export const Resource = Schema.Struct({
   name: Schema.String,
@@ -78,7 +96,7 @@ function createClient(directory: string) {
   const client: MCPClient = new Client(
     { name: "opencode", version: InstallationVersion },
     {
-      ...CLIENT_OPTIONS,
+      ...clientOptions(),
       listChanged: {
         tools: { autoRefresh: false, onChanged: (error) => client.onToolsChanged?.(error) },
       },
diff --git a/packages/opencode/test/mcp/catalog.test.ts b/packages/opencode/test/mcp/catalog.test.ts
index 3d77cb22b7..aa1543cec9 100644
--- a/packages/opencode/test/mcp/catalog.test.ts
+++ b/packages/opencode/test/mcp/catalog.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, test } from "bun:test"
 import { Client, InMemoryTransport } from "@modelcontextprotocol/client"
 import { Server } from "@modelcontextprotocol/server"
 import { McpCatalog } from "@/mcp/catalog"
+import { clientOptions } from "@/mcp"
 import { Effect } from "effect"
 
 const options = { toolCallId: "call_mcp", abortSignal: new AbortController().signal } as any
@@ -141,3 +142,60 @@ test("preserves output schema validation across paginated tool discovery", async
     await Promise.all([client.close(), server.close()])
   }
 })
+
+test("accepts and validates draft-07 tool output schemas", async () => {
+  const server = new Server({ name: "draft-07", version: "1.0.0" }, { capabilities: { tools: {} } })
+  let calls = 0
+  server.setRequestHandler("tools/list", () =>
+    Promise.resolve({
+      tools: [
+        {
+          name: "draft-07-tool",
+          inputSchema: { type: "object" as const },
+          outputSchema: {
+            $schema: "http://json-schema.org/draft-07/schema#",
+            type: "object" as const,
+            properties: { value: { type: "string" } },
+            required: ["value"],
+          },
+        },
+      ],
+    }),
+  )
+  server.setRequestHandler("tools/call", () => {
+    calls++
+    return Promise.resolve({ content: [], structuredContent: { value: calls === 1 ? "valid" : 42 } })
+  })
+
+  const client = new Client({ name: "draft-07-test", version: "1.0.0" }, clientOptions())
+  const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair()
+  await Promise.all([client.connect(clientTransport), server.connect(serverTransport)])
+
+  try {
+    const tools = await Effect.runPromise(McpCatalog.defs(client))
+    expect(tools?.map((tool) => tool.name)).toEqual(["draft-07-tool"])
+    await expect(client.callTool({ name: "draft-07-tool", arguments: {} })).resolves.toMatchObject({
+      structuredContent: { value: "valid" },
+    })
+    await expect(client.callTool({ name: "draft-07-tool", arguments: {} })).rejects.toThrow(/output schema/i)
+    expect(calls).toBe(2)
+  } finally {
+    await Promise.all([client.close(), server.close()])
+  }
+})
+
+test("isolates output schema caches between MCP clients", () => {
+  const stringSchema = {
+    $schema: "http://json-schema.org/draft-07/schema#",
+    $id: "https://example.com/shared-schema",
+    type: "string",
+  }
+  const numberSchema = { ...stringSchema, type: "number" }
+  const stringValidator = clientOptions().jsonSchemaValidator!.getValidator(stringSchema)
+  const numberValidator = clientOptions().jsonSchemaValidator!.getValidator(numberSchema)
+
+  expect(stringValidator("value").valid).toBe(true)
+  expect(stringValidator(42).valid).toBe(false)
+  expect(numberValidator(42).valid).toBe(true)
+  expect(numberValidator("value").valid).toBe(false)
+})