diff --git a/.changeset/reject-duplicate-tool-names.md b/.changeset/reject-duplicate-tool-names.md
new file mode 100644
index 00000000..574b11d7
--- /dev/null
+++ b/.changeset/reject-duplicate-tool-names.md
@@ -0,0 +1,5 @@
+---
+'@openrouter/agent': patch
+---
+
+`callModel` now throws when two client tools in `tools` share a name. Tool calls are dispatched by name, so with a duplicate the first tool in the list received the call, including a call that was approved while a different tool with that name was registered. Rename one of the tools, or pass `toolNamePrefix` to `createMCPTools` when combining MCP tools with local tools.
diff --git a/packages/agent/src/lib/model-result.ts b/packages/agent/src/lib/model-result.ts
index 166fb96f..466aaa77 100644
--- a/packages/agent/src/lib/model-result.ts
+++ b/packages/agent/src/lib/model-result.ts
@@ -98,6 +98,7 @@ import { ToolEventBroadcaster } from './tool-event-broadcaster.js';
 import type { AsyncToolInvocation } from './tool-executor.js';
 import {
   applyOnResponseReceivedHooks,
+  assertUniqueClientToolNames,
   executeTool,
   isAsyncToolInvocation,
   validateToolInput,
@@ -798,6 +799,8 @@ export class ModelResult<
     this.doomLoopMonitor = doomLoopConfig ? new DoomLoopMonitor(doomLoopConfig) : null;
     this.doomLoopEscalation = doomLoopConfig?.escalation ?? null;
 
+    assertUniqueClientToolNames(options.tools);
+
     // Runtime validation: approval decisions require state
     const hasApprovalDecisions =
       (options.approveToolCalls && options.approveToolCalls.length > 0) ||
diff --git a/packages/agent/src/lib/tool-executor.ts b/packages/agent/src/lib/tool-executor.ts
index 780917c9..c34bab16 100644
--- a/packages/agent/src/lib/tool-executor.ts
+++ b/packages/agent/src/lib/tool-executor.ts
@@ -18,6 +18,7 @@ import type {
   UnifiedTool,
 } from './tool-types.js';
 import {
+  findDuplicateClientToolName,
   hasExecuteFunction,
   isDeferredHandle,
   isGeneratorTool,
@@ -783,6 +784,24 @@ export function findToolByName(tools: Tool[], name: string): ClientTool | undefi
   );
 }
 
+/**
+ * Throw when two client tools share a name. Calls are dispatched by name, so a
+ * duplicate would let whichever tool is listed first receive arguments the
+ * model (or an approver) meant for the other.
+ */
+export function assertUniqueClientToolNames(tools: readonly Tool[] | undefined): void {
+  if (!tools) {
+    return;
+  }
+  const name = findDuplicateClientToolName(tools);
+  if (name !== undefined) {
+    throw new Error(
+      `Duplicate tool name "${name}": every tool passed to callModel must have a unique name. ` +
+        'Rename one, or use toolNamePrefix when combining MCP tools with local tools.',
+    );
+  }
+}
+
 /**
  * Format tool execution result as a string for sending to the model
  */
diff --git a/packages/agent/src/lib/tool-types.ts b/packages/agent/src/lib/tool-types.ts
index 41217cf8..3abb0cc3 100644
--- a/packages/agent/src/lib/tool-types.ts
+++ b/packages/agent/src/lib/tool-types.ts
@@ -1052,6 +1052,25 @@ export function isServerTool(tool: Tool): tool is ServerToolBase {
   return tool._brand === 'server-tool';
 }
 
+/**
+ * Return the first client tool name that appears more than once, or
+ * `undefined`. Server tools have no client name and are skipped.
+ */
+export function findDuplicateClientToolName(tools: readonly Tool[]): string | undefined {
+  const seen = new Set<string>();
+  for (const tool of tools) {
+    if (isServerTool(tool)) {
+      continue;
+    }
+    const name = tool.function.name;
+    if (seen.has(name)) {
+      return name;
+    }
+    seen.add(name);
+  }
+  return undefined;
+}
+
 /**
  * A client tool additionally branded as originating from an MCP server. The
  * `_mcp` marker is purely informational: it does NOT change how the tool is
diff --git a/packages/agent/src/mcp/build-tools.ts b/packages/agent/src/mcp/build-tools.ts
index 2eaaa137..f7e8d3ba 100644
--- a/packages/agent/src/mcp/build-tools.ts
+++ b/packages/agent/src/mcp/build-tools.ts
@@ -1,5 +1,6 @@
 import type { Client } from '@modelcontextprotocol/client';
 import type { Tool, ToolLoopKey } from '../lib/tool-types.js';
+import { findDuplicateClientToolName } from '../lib/tool-types.js';
 import { MCPError } from './errors.js';
 import { buildResourceTools } from './resource-tools.js';
 import type { UnconvertibleSchemaMode } from './schema/json-schema-to-zod.js';
@@ -93,25 +94,11 @@ export function buildTools(options: BuildToolsOptions): Tool[] {
   return tools;
 }
 
-function toolName(tool: Tool): string | undefined {
-  if ('function' in tool && typeof tool.function.name === 'string') {
-    return tool.function.name;
-  }
-  return undefined;
-}
-
 function assertNoDuplicateNames(tools: readonly Tool[]): void {
-  const seen = new Set<string>();
-  for (const tool of tools) {
-    const name = toolName(tool);
-    if (name === undefined) {
-      continue;
-    }
-    if (seen.has(name)) {
-      throw new MCPError(
-        `Duplicate tool name "${name}". Use toolNamePrefix or excludeTools to disambiguate.`,
-      );
-    }
-    seen.add(name);
+  const name = findDuplicateClientToolName(tools);
+  if (name !== undefined) {
+    throw new MCPError(
+      `Duplicate tool name "${name}". Use toolNamePrefix or excludeTools to disambiguate.`,
+    );
   }
 }
diff --git a/packages/agent/tests/unit/duplicate-tool-names.test.ts b/packages/agent/tests/unit/duplicate-tool-names.test.ts
new file mode 100644
index 00000000..f510003c
--- /dev/null
+++ b/packages/agent/tests/unit/duplicate-tool-names.test.ts
@@ -0,0 +1,193 @@
+import type { OpenRouterCore } from '@openrouter/sdk/core';
+import type * as models from '@openrouter/sdk/models';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { z } from 'zod/v4';
+import { tool } from '../../src/lib/tool.js';
+import type { ConversationState, StateAccessor, Tool } from '../../src/lib/tool-types.js';
+
+const mockBetaResponsesSend = vi.hoisted(() => vi.fn());
+
+vi.mock('@openrouter/sdk/funcs/betaResponsesSend', () => ({
+  betaResponsesSend: mockBetaResponsesSend,
+}));
+
+const { ModelResult } = await import('../../src/lib/model-result.js');
+const { callModel } = await import('../../src/inner-loop/call-model.js');
+
+function makeResponse(
+  id: string,
+  output: models.OpenResponsesResult['output'],
+): models.OpenResponsesResult {
+  return {
+    id,
+    object: 'response',
+    createdAt: 0,
+    model: 'test-model',
+    status: 'completed',
+    completedAt: 0,
+    output,
+    error: null,
+    incompleteDetails: null,
+    temperature: null,
+    topP: null,
+    presencePenalty: null,
+    frequencyPenalty: null,
+    metadata: null,
+    instructions: null,
+    tools: [],
+    toolChoice: 'auto',
+    parallelToolCalls: false,
+  };
+}
+
+function createMemoryAccessor<TTools extends readonly Tool[]>(): {
+  accessor: StateAccessor<TTools>;
+  get: () => ConversationState<TTools> | null;
+} {
+  let state: ConversationState<TTools> | null = null;
+  return {
+    accessor: {
+      load: async () => state,
+      save: async (next) => {
+        state = next;
+      },
+    },
+    get: () => state,
+  };
+}
+
+const request = {
+  model: 'test-model',
+  input: 'read the value',
+};
+
+function makeTool(
+  name: string,
+  execute: (input: { slot: string }) => Promise<{
+    ok: boolean;
+  }>,
+) {
+  return tool({
+    name,
+    inputSchema: z.object({
+      slot: z.string(),
+    }),
+    outputSchema: z.object({
+      ok: z.boolean(),
+    }),
+    requireApproval: true,
+    execute,
+  });
+}
+
+describe('duplicate client tool names', () => {
+  beforeEach(() => {
+    mockBetaResponsesSend.mockReset();
+  });
+
+  it('rejects a tools list where two client tools share a name', () => {
+    const first = makeTool('read_value', async () => ({
+      ok: true,
+    }));
+    const second = makeTool('read_value', async () => ({
+      ok: true,
+    }));
+
+    expect(
+      () =>
+        new ModelResult({
+          request,
+          client: {} as OpenRouterCore,
+          tools: [
+            first,
+            second,
+          ],
+        }),
+    ).toThrow(/Duplicate tool name "read_value"/);
+    expect(() =>
+      callModel({} as OpenRouterCore, {
+        ...request,
+        tools: [
+          first,
+          second,
+        ],
+      }),
+    ).toThrow(/Duplicate tool name "read_value"/);
+  });
+
+  it('does not run an approved call on a different same-named tool added before resume', async () => {
+    const approvedExecute = vi.fn(async () => ({
+      ok: true,
+    }));
+    const otherExecute = vi.fn(async () => ({
+      ok: true,
+    }));
+    const approved = makeTool('read_value', approvedExecute);
+    const other = makeTool('read_value', otherExecute);
+
+    mockBetaResponsesSend.mockResolvedValueOnce({
+      ok: true,
+      value: makeResponse('resp_pause', [
+        {
+          type: 'function_call',
+          id: 'fc_call_1',
+          callId: 'call_1',
+          name: 'read_value',
+          arguments: '{"slot":"synthetic-marker"}',
+          status: 'completed',
+        },
+      ]),
+    });
+    const memory = createMemoryAccessor<readonly Tool[]>();
+    await new ModelResult<readonly Tool[]>({
+      request,
+      client: {} as OpenRouterCore,
+      tools: [
+        approved,
+      ],
+      state: memory.accessor,
+    }).getResponse();
+    expect(memory.get()?.status).toBe('awaiting_approval');
+
+    mockBetaResponsesSend.mockResolvedValue({
+      ok: true,
+      value: makeResponse('resp_done', [
+        {
+          type: 'message',
+          id: 'msg_1',
+          role: 'assistant',
+          status: 'completed',
+          content: [
+            {
+              type: 'output_text',
+              text: 'Done.',
+              annotations: [],
+            },
+          ],
+        },
+      ]),
+    });
+
+    let resumeError: unknown;
+    try {
+      await new ModelResult<readonly Tool[]>({
+        request,
+        client: {} as OpenRouterCore,
+        tools: [
+          other,
+          approved,
+        ],
+        state: memory.accessor,
+        approveToolCalls: [
+          'call_1',
+        ],
+      }).getResponse();
+    } catch (error) {
+      resumeError = error;
+    }
+
+    expect(otherExecute).not.toHaveBeenCalled();
+    expect(String(resumeError)).toMatch(/Duplicate tool name "read_value"/);
+    expect(memory.get()?.status).toBe('awaiting_approval');
+  });
+});