diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index b3265d801..cefb1f32a 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -10,6 +10,10 @@
 
 - Changed OAuth browser pages to use the color Pi logo.
 
+### Fixed
+
+- Fixed OpenAI Responses requests failing with `Expected an ID that begins with 'ctc'` when replaying grammar tool calls, such as `codemode`, from another provider or a gateway like Radius.
+
 ## [0.99.2] - 2026-09-30
 
 ### Added
diff --git a/packages/ai/src/api/openai-responses-shared.ts b/packages/ai/src/api/openai-responses-shared.ts
index 54abbdc8f..59c9b5b42 100644
--- a/packages/ai/src/api/openai-responses-shared.ts
+++ b/packages/ai/src/api/openai-responses-shared.ts
@@ -294,14 +294,13 @@ export function convertResponsesMessages<TApi extends Api>(
 					let itemId: string | undefined = itemIdRaw;
 
 					// For different-model messages, set id to undefined to avoid pairing validation.
-					// OpenAI tracks which fc_xxx IDs were paired with rs_xxx reasoning items.
+					// OpenAI tracks which item IDs were paired with rs_xxx reasoning items.
 					// By omitting the id, we avoid triggering that validation (like cross-provider does).
-					// When replaying custom-tool calls as a function_call, also drop non-fc_* ids such as
-					// ctc_* custom-tool ids because function_call item ids must be fc_*.
-					if (
-						(isDifferentModel && itemId?.startsWith("fc_")) ||
-						(customInputProperty === undefined && !itemId?.startsWith("fc_"))
-					) {
+					// Also drop ids that do not match the replayed item type: function_call ids must be fc_*
+					// and custom_tool_call ids must be ctc_*. Foreign tool call ids are normalized to fc_*, and
+					// a call can switch between the two types when grammar tool support differs.
+					const itemIdPrefix = customInputProperty === undefined ? "fc_" : "ctc_";
+					if (isDifferentModel || !itemId?.startsWith(itemIdPrefix)) {
 						itemId = undefined;
 					}
 
diff --git a/packages/ai/test/constrained-sampling.test.ts b/packages/ai/test/constrained-sampling.test.ts
index b3ce0ad6a..8eb9ea8da 100644
--- a/packages/ai/test/constrained-sampling.test.ts
+++ b/packages/ai/test/constrained-sampling.test.ts
@@ -256,6 +256,40 @@ describe("constrained tool sampling", () => {
 		});
 	});
 
+	// earendil-works/radius#115: a gateway forwards another model's history as a foreign provider.
+	it("drops foreign item ids when replaying grammar calls as custom Responses items", () => {
+		const context = normalizeContext({
+			messages: [
+				{
+					role: "assistant",
+					api: "pi-messages",
+					provider: "radius",
+					model: "gpt-other",
+					content: [{ type: "toolCall", id: "call_1|ctc_1", name: "sample_tool", arguments: { payload: "abc" } }],
+					usage: makeUsage(),
+					stopReason: "toolUse",
+					timestamp: Date.now(),
+				},
+				{
+					role: "toolResult",
+					toolCallId: "call_1|ctc_1",
+					toolName: "sample_tool",
+					content: [{ type: "text", text: "done" }],
+					isError: false,
+					timestamp: Date.now(),
+				},
+			],
+		});
+
+		const messages = convertResponsesMessages(makeModel(), context, new Set(["openai"]), {
+			grammarToolInputProperties: new Map([["sample_tool", "payload"]]),
+		});
+
+		const call = messages.find((item) => "type" in item && item.type === "custom_tool_call");
+		expect(call).toMatchObject({ type: "custom_tool_call", call_id: "call_1", input: "abc" });
+		expect(call && "id" in call ? call.id : undefined).toBeUndefined();
+	});
+
 	it("keeps grammar input JSON deltas append-only", () => {
 		const buffer = { input: "", started: false, closed: false };
 		const first = appendGrammarToolInputJsonDelta(buffer, "payload", 'a"', false);