diff --git a/packages/translate/__tests__/composition/request/tool-names_test.ts b/packages/translate/__tests__/composition/request/tool-names_test.ts
index 2ce4538ba7..6190eb8164 100644
--- a/packages/translate/__tests__/composition/request/tool-names_test.ts
+++ b/packages/translate/__tests__/composition/request/tool-names_test.ts
@@ -109,3 +109,8 @@ test.each([undefined, 'files'])('allocates distinct names per callable kind in n
   assertEquals([...selected.context.customToolNames], [second]);
   assertEquals(selected.context.originalToolNames.get(second), { name: 'edit', ...(namespace === undefined ? {} : { namespace }) });
 });
+
+test('a request declaring one callable identity twice is refused before allocation', () => {
+  const tool: IRInputTool = { type: 'function', namespace: 'agents', name: 'wait', parameters: { type: 'object' } };
+  assertThrows(() => allocateToolNames(request([tool, tool]), []), TranslateInputException, 'repeated declarations');
+});
diff --git a/packages/translate/__tests__/protocols/openai-responses/as-source/allowed-tools_test.ts b/packages/translate/__tests__/protocols/openai-responses/as-source/allowed-tools_test.ts
index 390c27f49d..6926df0a01 100644
--- a/packages/translate/__tests__/protocols/openai-responses/as-source/allowed-tools_test.ts
+++ b/packages/translate/__tests__/protocols/openai-responses/as-source/allowed-tools_test.ts
@@ -202,3 +202,33 @@ for (const allowed of [false, true]) {
       : { type: 'function', function: { name: expected } });
   });
 }
+
+test.each(['chat', 'messages'] as const)('redeclared callables reach %s once with the latest declaration in the first slot', async target => {
+  const tools = (description: string): OpenAIResponsesTool[] => [
+    { type: 'namespace', name: 'agents', description: '', tools: [{ type: 'function', name: 'wait', parameters: { type: 'object' } }] },
+    { type: 'function', name: 'exec', description, parameters: { type: 'object' } },
+  ];
+  const payload: OpenAIResponsesRequestPayloadEx = {
+    model: 'm', stream: true,
+    tools: [...tools('first'), { type: 'function', name: 'read', parameters: { type: 'object' } }],
+    input: [
+      { type: 'additional_tools', role: 'developer', tools: tools('second') },
+      { type: 'message', role: 'user', content: 'hi' },
+      { type: 'additional_tools', role: 'developer', tools: tools('latest') },
+    ],
+    tool_choice: { type: 'allowed_tools', mode: 'auto', tools: [{ type: 'namespace', name: 'agents' }] },
+  };
+  const codec = createTestSidecarCodec();
+  if (target === 'chat') {
+    const { target: wire } = await buildChat(payload, codec);
+    assertEquals(wire.tools?.map(tool => tool.type === 'function' ? [tool.function.name, tool.function.description] : []), [
+      ['agents_wait', undefined], ['exec', 'latest'], ['read', undefined],
+    ]);
+    assertEquals(wire.tool_choice, { type: 'allowed_tools', allowed_tools: { mode: 'auto', tools: [{ type: 'function', function: { name: 'agents_wait' } }] } });
+  } else {
+    const { target: wire } = await buildMessages({ ...payload, tool_choice: 'auto' }, codec);
+    assertEquals(wire.tools?.map(tool => 'name' in tool ? [tool.name, 'description' in tool ? tool.description : undefined] : []), [
+      ['agents_wait', undefined], ['exec', 'latest'], ['read', undefined],
+    ]);
+  }
+});
diff --git a/packages/translate/src/composition/request/tool-names.ts b/packages/translate/src/composition/request/tool-names.ts
index e121ead99f..8685765188 100644
--- a/packages/translate/src/composition/request/tool-names.ts
+++ b/packages/translate/src/composition/request/tool-names.ts
@@ -28,6 +28,13 @@ export const allocateToolNames = <Restored>(
   request: IRInputRequestHeader,
   inputTurns: readonly (IRInputTurnWithCallIds | RestoredAssistantTurn<Restored>)[],
 ): { readonly request: IRInputRequestHeader; readonly turns: readonly (IRInputTurnWithCallIds | RestoredAssistantTurn<Restored>)[]; readonly context: ToolNameContext } => {
+  const declared = new Set<string>();
+  for (const tool of request.tools ?? []) {
+    const key = callableKey(tool);
+    // One identity allocates one name, and these wires reject a repeated declaration name.
+    if (declared.has(key)) throw new TranslateInputException(`Cannot translate repeated declarations of tool '${tool.name}'.`);
+    declared.add(key);
+  }
   const callables = [
     ...(request.tools ?? []),
     ...inputTurns.flatMap(turn => turn.kind === 'restored' ? [] : turn.items.flatMap(item =>
diff --git a/packages/translate/src/ir/input/types.ts b/packages/translate/src/ir/input/types.ts
index 289de14082..55dccd9e25 100644
--- a/packages/translate/src/ir/input/types.ts
+++ b/packages/translate/src/ir/input/types.ts
@@ -54,7 +54,8 @@ export interface IRInputOutputFormat {
 // All
 export interface IRInputRequestHeader {
   readonly settings: IRInputSettings; // All
-  // Absent and empty are the same thing: a request that declares no tool.
+  // Absent and empty are the same thing: a request that declares no tool. Each
+  // callable identity appears once; a source that allows redeclaration resolves it.
   readonly tools?: readonly IRInputTool[]; // All
   readonly tool_choice?: IRInputToolChoice; // All
 }
diff --git a/packages/translate/src/protocols/openai-responses/as-source/input-to.ts b/packages/translate/src/protocols/openai-responses/as-source/input-to.ts
index 37f21fa612..b8a494a84d 100644
--- a/packages/translate/src/protocols/openai-responses/as-source/input-to.ts
+++ b/packages/translate/src/protocols/openai-responses/as-source/input-to.ts
@@ -228,14 +228,10 @@ const selectorOf = (payload: CanonicalOpenAIResponsesPayload, selector: Record<s
   if (typeof selector.name !== 'string' || Object.keys(selector).some(key => key !== 'type' && key !== 'name')) {
     throw new TranslateInputException('Cannot translate namespace selector extensions.');
   }
-  const groups = [
-    ...(payload.tools ?? []),
-    ...payload.input.flatMap(item => item.type === 'additional_tools' || item.type === 'tool_search_output' ? item.tools : []),
-  ].flatMap(tool => tool.type === 'namespace' && tool.name === selector.name ? [tool] : []);
-  if (groups.length === 0) throw new TranslateInputException(`Cannot translate group selector '${String(selector.name)}' without a matching declaration.`);
-  return groups.flatMap(group => group.tools.flatMap(child => child.type === 'function' || child.type === 'custom'
-    ? [{ type: child.type, name: child.name, namespace: group.name } as IRInputToolSelector]
-    : []));
+  if (!toolInventories(payload).some(tool => tool.type === 'namespace' && tool.name === selector.name)) {
+    throw new TranslateInputException(`Cannot translate group selector '${String(selector.name)}' without a matching declaration.`);
+  }
+  return declarations.flatMap(({ type, namespace, name }) => namespace === selector.name ? [{ type, namespace, name }] : []);
 };
 
 const toolChoiceToIR = (
@@ -296,14 +292,22 @@ const toolGroupToIR = (tool: OpenAIResponsesTool): IRInputTool[] => {
 
 // A request can defer declarations to an inventory item inside the
 // conversation; those are tools like any other, not things the model said.
-const deferredInventories = (payload: CanonicalOpenAIResponsesPayload): OpenAIResponsesTool[] =>
-  payload.input.flatMap(item => item.type === 'additional_tools' || item.type === 'tool_search_output' ? item.tools : []);
+const toolInventories = (payload: CanonicalOpenAIResponsesPayload): OpenAIResponsesTool[] => [
+  ...(payload.tools ?? []),
+  ...payload.input.flatMap(item => item.type === 'additional_tools' || item.type === 'tool_search_output' ? item.tools : []),
+];
+
+// Codex declares a namespace in `tools`, again in `additional_tools`, and once
+// more after each compaction, so one callable can arrive many times. The latest
+// declaration is the caller's current statement of it; it takes the slot of the
+// first, which keeps the declared prefix stable for prompt caching.
+const latestDeclarations = (tools: readonly IRInputTool[]): IRInputTool[] =>
+  [...new Map(tools.map(tool => [JSON.stringify([tool.type, tool.namespace, tool.name]), tool])).values()];
 
 const openaiResponsesToolsToIR = (payload: CanonicalOpenAIResponsesPayload): Pick<IRInputRequestHeader, 'tools' | 'tool_choice'> => {
-  const declared = payload.tools?.flatMap(toolGroupToIR) ?? [];
-  const deferred = deferredInventories(payload).flatMap(toolGroupToIR);
-  const tools = declared.length === 0 && deferred.length === 0 ? undefined : [...declared, ...deferred];
-  const toolChoice = payload.tool_choice == null ? undefined : toolChoiceToIR(payload, payload.tool_choice, tools ?? []);
+  const declarations = latestDeclarations(toolInventories(payload).flatMap(toolGroupToIR));
+  const tools = declarations.length === 0 ? undefined : declarations;
+  const toolChoice = payload.tool_choice == null ? undefined : toolChoiceToIR(payload, payload.tool_choice, declarations);
   return {
     ...(tools === undefined ? {} : { tools }),
     ...(toolChoice === undefined ? {} : { tool_choice: toolChoice }),