diff --git a/services/agent-harness/src/tools/mcp.test.ts b/services/agent-harness/src/tools/mcp.test.ts
new file mode 100644
index 0000000000..5c41eaf0df
--- /dev/null
+++ b/services/agent-harness/src/tools/mcp.test.ts
@@ -0,0 +1,198 @@
+import { expect, it } from 'vitest';
+import type { Tool } from '@modelcontextprotocol/sdk/types.js';
+import { RunLimitsSchema } from '../commands';
+import { executeMcp } from './mcp';
+import { mcpTestFixture } from './mcp-test-fixture';
+import { toolDefinitions } from '@kilocode/agent-harness/tools';
+import { ToolCallSchema } from '@kilocode/agent-harness/contracts';
+import { evaluateDispatch } from '@kilocode/agent-harness/policy';
+
+it('requires Ask approval despite a discovered read-only hint', async () => {
+  const request = await fixture().call();
+  const definition = toolDefinitions.find(tool => tool.name === request.name);
+  const call = ToolCallSchema.parse({
+    id: crypto.randomUUID(),
+    runId: crypto.randomUUID(),
+    name: request.name,
+    definitionVersion: definition?.version,
+    arguments: request.arguments,
+    context: { type: 'personal' },
+    effect: definition?.effect,
+    executionTarget: { kind: 'backend' },
+    approval: null,
+    state: 'pending',
+    result: null,
+  });
+  expect(
+    evaluateDispatch(call, call, {
+      permissionMode: 'ask',
+      permissionRevision: 0,
+      expectedPermissionRevision: 0,
+      authorized: true,
+      available: true,
+      clientReady: true,
+      questionAnswered: true,
+      trustedRead: true,
+    })
+  ).toBe('approval');
+});
+
+function fixture() {
+  const { key, state, connection, fetchImpl } = mcpTestFixture();
+  const run = (request: unknown = { name: 'mcp.discover', arguments: {} }, limits = {}) =>
+    executeMcp(
+      request,
+      [connection],
+      { deadline: Date.now() + 1000, limits: RunLimitsSchema.parse(limits) },
+      fetchImpl
+    );
+  const call = async () => {
+    const result = await run();
+    expect(result.status).toBe('succeeded');
+    expect(JSON.stringify(result)).not.toContain('derived-secret');
+    const [found] = (result as { output: any[] }).output;
+    return {
+      name: 'mcp.call',
+      arguments: {
+        serverId: found.serverId,
+        configurationVersion: found.configurationVersion,
+        definitionVersion: found.definitionVersion,
+        name: found.name,
+        arguments: { [key]: 2 },
+      },
+    };
+  };
+  return { key, state, connection, run, call };
+}
+
+it('discovers runtime-only schemas and returns validated business data without authorization', async () => {
+  const f = fixture();
+  const call = await f.call();
+  const result = await f.run(call);
+  expect(result).toEqual({ status: 'succeeded', output: f.state.result });
+  expect(f.state.effects).toEqual([call.arguments.arguments]);
+  expect(JSON.stringify(result)).not.toContain('derived-secret');
+});
+it.each(['missing', 'wrong_type', 'minimum', 'extra'])(
+  'rejects malformed arguments before an effect: %s',
+  async mode => {
+    const f = fixture(),
+      call = await f.call();
+    const key = Object.keys(call.arguments.arguments)[0];
+    const args =
+      mode === 'missing'
+        ? {}
+        : mode === 'extra'
+          ? { ...call.arguments.arguments, extra: true }
+          : { [key]: mode === 'minimum' ? 0 : 'two' };
+    expect(
+      await f.run({ ...call, arguments: { ...call.arguments, arguments: args } })
+    ).toMatchObject({ status: 'failed', error: { message: 'invalid_input' } });
+    expect(f.state.effects).toEqual([]);
+  }
+);
+it.each(['inputSchema', 'outputSchema'] as const)(
+  'rejects unsupported references and malformed %s before discovery or effects',
+  async field => {
+    for (const extra of [
+      { $ref: 'https://evil.example/schema' },
+      { $ref: '#/properties/value' },
+      { required: 'wrong' },
+      { required: ['value', 'value'] },
+      { unevaluatedProperties: false },
+      { properties: { value: { type: 'unsupported' } } },
+    ]) {
+      const f = fixture(),
+        call = await f.call();
+      f.state.tool[field] = { type: 'object', ...extra } as Tool['inputSchema'];
+      for (const request of [undefined, call])
+        expect(await f.run(request), JSON.stringify(extra)).toMatchObject({
+          status: 'failed',
+          error: { message: 'invalid_schema', retryable: false },
+        });
+      expect(f.state.effects).toEqual([]);
+    }
+  }
+);
+it.each([
+  [401, 'reauthorization_required', true],
+  [403, 'reauthorization_required', true],
+  [503, 'unavailable_server', true],
+  [302, 'unsafe_destination', false],
+  [400, 'unsafe_destination', false],
+] as const)('keeps HTTP failure %i distinct and sanitized', async (status, reason, retryable) => {
+  const f = fixture();
+  f.state.status = status;
+  expect(await f.run()).toMatchObject({ status: 'failed', error: { message: reason, retryable } });
+  expect(JSON.stringify(await f.run())).not.toContain('provider-secret');
+});
+it.each(['description', 'inputSchema', 'outputSchema'] as const)(
+  'requires a new call after %s or configuration changes',
+  async field => {
+    const f = fixture(),
+      call = await f.call();
+    expect((await f.call()).arguments).toEqual(call.arguments);
+    f.state.tool = {
+      ...f.state.tool,
+      [field]:
+        field === 'description'
+          ? 'Changed operation'
+          : { ...f.state.tool[field], additionalProperties: true },
+    };
+    expect(await f.run(call)).toMatchObject({
+      status: 'failed',
+      error: { message: 'definition_changed' },
+    });
+    f.connection.configurationVersion = '2';
+    expect(await f.run(call)).toMatchObject({
+      status: 'failed',
+      error: { message: 'definition_changed' },
+    });
+    expect(f.state.effects).toEqual([]);
+    expect(await f.run(await f.call())).toMatchObject({ status: 'succeeded' });
+  }
+);
+it.each([
+  ['invalid', 'invalid_output'],
+  ['wrong_type', 'invalid_output'],
+  ['malformed', 'invalid_output'],
+  ['missing', 'invalid_output'],
+  ['error', 'invalid_output'],
+  ['lose', 'unavailable_server'],
+  ['overflow', 'limit_exceeded'],
+  ['stall', 'unavailable_server'],
+  ['businessLimit', 'limit_exceeded'],
+])('retains unknown outcomes without mutation replay: %s', async (mode, reason) => {
+  const f = fixture(),
+    call = await f.call();
+  if (mode === 'invalid') f.state.result = { content: [], structuredContent: { wrong: true } };
+  if (mode === 'wrong_type')
+    f.state.result = { content: [], structuredContent: { [f.key]: 'two' } };
+  if (mode === 'malformed') f.state.result = { content: 'provider-secret' };
+  if (mode === 'missing') f.state.result = { content: [] };
+  if (mode === 'error') f.state.result = { content: [], isError: true };
+  if (mode === 'businessLimit')
+    f.state.result = {
+      ...(f.state.result as object),
+      content: [{ type: 'text', text: 'é'.repeat(1600) }],
+    };
+  f.state.lose = mode === 'lose';
+  f.state.overflow = mode === 'overflow';
+  f.state.stall = mode === 'stall';
+  const result = await f.run(call, {
+    httpResponseBytes: mode === 'businessLimit' ? 8192 : 2048,
+    toolOutputBytes: 2048,
+    toolAttemptMs: 100,
+  });
+  expect(result).toEqual({ status: 'outcome_unknown', reason });
+  expect(f.state.effects).toEqual([call.arguments.arguments]);
+  expect(JSON.stringify(result)).not.toContain('provider-secret');
+});
+it('returns no invented definitions without configured connections', async () => {
+  expect(
+    await executeMcp({ name: 'mcp.discover', arguments: {} }, [], {
+      deadline: Date.now() + 1000,
+      limits: RunLimitsSchema.parse({}),
+    })
+  ).toEqual({ status: 'succeeded', output: [] });
+});
diff --git a/services/agent-harness/src/tools/mcp.ts b/services/agent-harness/src/tools/mcp.ts
new file mode 100644
index 0000000000..aef5025367
--- /dev/null
+++ b/services/agent-harness/src/tools/mcp.ts
@@ -0,0 +1,199 @@
+import { z } from 'zod';
+import { ErrorCode, McpError } from '@modelcontextprotocol/sdk/types.js';
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { CfWorkerJsonSchemaValidator } from '@modelcontextprotocol/sdk/validation/cfworker-provider.js';
+import type { JsonSchemaType } from '@modelcontextprotocol/sdk/validation/types.js';
+import { canonicalizeValidatedInput } from '@kilocode/agent-harness/commands';
+import type { ToolOutcome } from '@kilocode/agent-harness/contracts';
+import { toolDefinitions, ToolRequestSchema } from '@kilocode/agent-harness/tools';
+import { bytes, type RunLimits } from '../limits';
+import { createMcpTransportFactory, type McpConnection } from './mcp-transport';
+
+export type { McpConnection } from './mcp-transport';
+
+// Only this checked vocabulary is supported. References and unknown keywords fail closed.
+const unique = (values: readonly unknown[]) =>
+  new Set(values.map(canonicalizeValidatedInput)).size === values.length;
+const kind = z.enum(['object', 'array', 'string', 'number', 'integer', 'boolean', 'null']);
+const schema: z.ZodType<JsonSchemaType> = z.lazy(() =>
+  z.strictObject({
+    $schema: z.literal('https://json-schema.org/draft/2020-12/schema').optional(),
+    type: z.union([kind, z.array(kind).min(1).refine(unique)]).optional(),
+    properties: z.record(z.string(), schema).optional(),
+    required: z.array(z.string()).refine(unique).optional(),
+    additionalProperties: z.union([z.boolean(), schema]).optional(),
+    items: schema.optional(),
+    enum: z.array(z.json()).min(1).refine(unique).optional(),
+    const: z.json().optional(),
+    minimum: z.number().optional(),
+    maximum: z.number().optional(),
+    minLength: z.int().nonnegative().optional(),
+    maxLength: z.int().nonnegative().optional(),
+    minItems: z.int().nonnegative().optional(),
+    maxItems: z.int().nonnegative().optional(),
+    title: z.string().optional(),
+    description: z.string().optional(),
+    default: z.json().optional(),
+  })
+);
+const codes = {
+  unavailable_server: 'unavailable_tool',
+  reauthorization_required: 'reauthorization_required',
+  invalid_schema: 'invalid_output',
+  unsafe_destination: 'invalid_input',
+  definition_changed: 'invalid_input',
+  invalid_input: 'invalid_input',
+  invalid_output: 'invalid_output',
+  limit_exceeded: 'limit_exceeded',
+} as const;
+
+/** Connections contain ephemeral gateway authorization, never provider credentials or stored state. */
+export async function executeMcp(
+  input: unknown,
+  connections: readonly McpConnection[],
+  budget: { deadline: number; limits: RunLimits },
+  fetchImpl: typeof fetch = fetch
+): Promise<ToolOutcome> {
+  let reason: keyof typeof codes | undefined;
+  let dispatched = false;
+  const abort = new AbortController();
+  const stop = (value: keyof typeof codes): never => {
+    // Closing the transport on abort must not replace the original failure.
+    reason ??= value;
+    const error = new Error(reason);
+    abort.abort(error);
+    throw error;
+  };
+  const timeout = Math.min(budget.limits.toolAttemptMs, budget.deadline - Date.now());
+  const timer = setTimeout(() => abort.abort(), Math.max(0, timeout));
+  const options = {
+    signal: abort.signal,
+    timeout: Math.max(1, timeout),
+    resetTimeoutOnProgress: false,
+  };
+  const transportFor = createMcpTransportFactory(
+    abort.signal,
+    budget.limits.httpResponseBytes,
+    stop,
+    fetchImpl
+  );
+  const validator = new CfWorkerJsonSchemaValidator();
+  const checked = {
+    getValidator<T>(value: JsonSchemaType) {
+      try {
+        if (!schema.safeParse(value).success) stop('invalid_schema');
+        return validator.getValidator<T>(value);
+      } catch {
+        return stop('invalid_schema');
+      }
+    },
+  };
+  try {
+    if (timeout <= 0) stop('limit_exceeded');
+    const parsed = ToolRequestSchema.safeParse(input);
+    if (!parsed.success) stop('invalid_input');
+    const request = parsed.data;
+    if (request.name !== 'mcp.discover' && request.name !== 'mcp.call') stop('invalid_input');
+    const contract = toolDefinitions
+      .filter(tool => tool.group === 'mcp')
+      .find(tool => tool.name === request.name);
+    if (!contract) stop('invalid_input');
+    if (bytes(request.arguments) > budget.limits.toolInputBytes) stop('limit_exceeded');
+    const definitions = [];
+    for (const connection of connections) {
+      if (request.name === 'mcp.call' && request.arguments.serverId !== connection.serverId)
+        continue;
+      if (
+        request.name === 'mcp.call' &&
+        request.arguments.configurationVersion !== connection.configurationVersion
+      )
+        stop('definition_changed');
+      const transport = transportFor(connection);
+      const client = new Client(
+        { name: 'kilo-agent-harness', version: '1' },
+        { jsonSchemaValidator: checked }
+      );
+      try {
+        await client.connect(transport, options);
+        let cursor: string | undefined;
+        const seen = new Set<string>();
+        do {
+          const page = await client.listTools(cursor ? { cursor } : {}, options);
+          for (const tool of page.tools) {
+            if (!tool.outputSchema || tool.execution?.taskSupport === 'required')
+              stop('invalid_schema');
+            checked.getValidator(tool.inputSchema);
+            checked.getValidator(tool.outputSchema);
+            const digest = await crypto.subtle.digest(
+              'SHA-256',
+              new TextEncoder().encode(canonicalizeValidatedInput(tool))
+            );
+            const definition = {
+              serverId: connection.serverId,
+              configurationVersion: connection.configurationVersion,
+              name: tool.name,
+              definitionVersion: Array.from(new Uint8Array(digest), byte =>
+                byte.toString(16).padStart(2, '0')
+              ).join(''),
+              inputSchema: tool.inputSchema,
+              outputSchema: tool.outputSchema,
+            };
+            definitions.push(definition);
+            if (bytes(definitions) > budget.limits.toolOutputBytes) stop('limit_exceeded');
+            if (request.name !== 'mcp.call' || request.arguments.name !== tool.name) continue;
+            if (request.arguments.definitionVersion !== definition.definitionVersion)
+              stop('definition_changed');
+            if (!checked.getValidator(tool.inputSchema)(request.arguments.arguments).valid)
+              stop('invalid_input');
+            dispatched = true;
+            const result = await client.callTool(
+              { name: tool.name, arguments: request.arguments.arguments },
+              undefined,
+              options
+            );
+            if (
+              result.isError ||
+              !checked.getValidator(tool.outputSchema)(result.structuredContent).valid
+            )
+              stop('invalid_output');
+            const output = { content: result.content, structuredContent: result.structuredContent };
+            if (bytes(output) > budget.limits.toolOutputBytes) stop('limit_exceeded');
+            return { status: 'succeeded', output: contract.outputSchema.parse(output) };
+          }
+          cursor = page.nextCursor;
+          if (cursor && seen.has(cursor)) stop('invalid_schema');
+          if (cursor) seen.add(cursor);
+        } while (cursor);
+      } finally {
+        await client.close();
+      }
+    }
+    if (request.name === 'mcp.call') stop('unavailable_server');
+    return { status: 'succeeded', output: contract.outputSchema.parse(definitions) };
+  } catch (error) {
+    if (
+      (error instanceof McpError &&
+        [ErrorCode.InvalidParams, ErrorCode.InvalidRequest, ErrorCode.ParseError].includes(
+          error.code
+        )) ||
+      // SDK response parsing uses Zod Mini; the core error also covers Classic.
+      error instanceof z.core.$ZodError ||
+      error instanceof SyntaxError
+    )
+      reason ??= dispatched ? 'invalid_output' : 'invalid_schema';
+    reason ??= 'unavailable_server';
+    // No provider reconciliation contract exists here. A dispatched unknown effect is never replayed.
+    return dispatched
+      ? { status: 'outcome_unknown', reason }
+      : {
+          status: 'failed',
+          error: {
+            code: codes[reason],
+            message: reason,
+            retryable: reason === 'unavailable_server' || reason === 'reauthorization_required',
+          },
+        };
+  } finally {
+    clearTimeout(timer);
+  }
+}