diff --git a/CHANGELOG.md b/CHANGELOG.md
index cc88c90d7..b2562648b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
 
 ### Fixed
 
+- An OpenAI Responses stream that finishes with a tool call whose `output_item.done` never arrived now ends with an error instead of handing that call to the agent loop. Non-compliant servers that reuse or omit `output_index` (observed with OpenAI-compatible gateways) could otherwise produce duplicate or mixed-up tool calls in one assistant message, which ran the same tool twice and then failed every later turn on replay with `Duplicate value for 'tool_call_id'`.
 - Recover a normal assistant response that contains neither visible text nor an executable tool call: keep the original message in history unchanged, append one hidden user-role recovery prompt, and continue. Recovery is bounded by a streak of consecutive empty turns. Only a turn with visible text or an executed tool call resets it, and so does any user input, so a long autonomous run is not ended by one transient empty response and a provider error in between does not refill the budget. Automatic compaction runs first when the response also exceeded the context window.
 - A run that ends without a usable answer reports it on `agent_settled` as `failure` (reason and message), instead of rewriting the assistant message's provider stop reason, so a settled run no longer means success by itself. Subagent lanes and goal continuation act on it, and print/JSON pick their exit code from the same classification, so no consumer decides success from `stopReason` alone. `assistantTerminalFailure()` in `@step-harness/providers` applies the same rule to a message that has not settled yet. The hidden recovery message records the entry it answers in `details.recoversEntryId`.
 - Summary requests now use the provider's model output limit and remaining-context sizing instead of fixed compaction/branch caps or reserve-derived budgets. Existing reserve arguments remain accepted for call compatibility; automatic trigger and branch source-selection settings are unchanged.
diff --git a/packages/providers/src/api/openai-responses-shared.ts b/packages/providers/src/api/openai-responses-shared.ts
index ae9745361..3ea14cbe8 100644
--- a/packages/providers/src/api/openai-responses-shared.ts
+++ b/packages/providers/src/api/openai-responses-shared.ts
@@ -758,6 +758,21 @@ export async function processResponsesStream<TApi extends Api>(
 	if (!sawTerminalResponseEvent) {
 		throw new Error("OpenAI Responses stream ended before a terminal response event");
 	}
+	// The agent runs every tool call in the final message. Refuse to hand over calls whose
+	// output_item.done never arrived: their arguments may be cut off or mixed up, e.g. when a
+	// non-compliant server reuses or omits output_index. Finalized calls drop their scratch
+	// buffers, so a surviving buffer means this call never completed.
+	if (output.stopReason === "toolUse") {
+		for (const block of output.content) {
+			if (block.type !== "toolCall") continue;
+			const toolCall = block as StreamingToolCall;
+			if (toolCall.partialJson !== undefined || toolCall.customInput !== undefined) {
+				throw new Error(
+					`OpenAI Responses stream completed with an unfinished tool call: ${toolCall.name} (${toolCall.id})`,
+				);
+			}
+		}
+	}
 }
 
 function mapStopReason(
diff --git a/packages/providers/test/openai-responses-terminal-event.test.ts b/packages/providers/test/openai-responses-terminal-event.test.ts
index a6c2fd94c..f4020a2f6 100644
--- a/packages/providers/test/openai-responses-terminal-event.test.ts
+++ b/packages/providers/test/openai-responses-terminal-event.test.ts
@@ -203,6 +203,70 @@ async function* createPhasedMessageEvents(
 	} as ResponseStreamEvent;
 }
 
+// A server that omits output_index entirely, then sends both done events after all deltas.
+async function* createToolCallsWithoutOutputIndexEvents(): AsyncIterable<ResponseStreamEvent> {
+	const call = (n: string) => ({ type: "function_call", id: `fc_${n}`, call_id: `call_${n}`, name: "bash" });
+	const events = [
+		{ type: "response.output_item.added", item: { ...call("a"), arguments: "" } },
+		{ type: "response.function_call_arguments.delta", item_id: "fc_a", delta: '{"command":"echo a"}' },
+		{ type: "response.output_item.added", item: { ...call("b"), arguments: "" } },
+		{ type: "response.function_call_arguments.delta", item_id: "fc_b", delta: '{"command":"echo b"}' },
+		{ type: "response.output_item.done", item: { ...call("a"), arguments: '{"command":"echo a"}' } },
+		{ type: "response.output_item.done", item: { ...call("b"), arguments: '{"command":"echo b"}' } },
+		{ type: "response.completed", response: { id: "resp_no_output_index", status: "completed" } },
+	];
+	for (const event of events) yield event as unknown as ResponseStreamEvent;
+}
+
+// A streamed call whose output_item.done never arrives before the terminal event.
+async function* createUnfinishedToolCallEvents(): AsyncIterable<ResponseStreamEvent> {
+	yield {
+		type: "response.output_item.added",
+		sequence_number: 0,
+		output_index: 0,
+		item: { type: "function_call", id: "fc_1", call_id: "call_1", name: "bash", arguments: "" },
+	} as ResponseStreamEvent;
+	yield {
+		type: "response.function_call_arguments.delta",
+		sequence_number: 1,
+		output_index: 0,
+		item_id: "fc_1",
+		delta: '{"command":"rm -rf /tmp/build',
+	} as ResponseStreamEvent;
+	yield {
+		type: "response.completed",
+		sequence_number: 2,
+		response: { id: "resp_unfinished", status: "completed" },
+	} as ResponseStreamEvent;
+}
+
+// A server that reuses an already-closed output_index for a later item, then re-sends the
+// earlier call's output_item.done. The slot map rebuilds that call, which leaves a second
+// content block carrying the same tool call id.
+async function* createReusedOutputIndexEvents(): AsyncIterable<ResponseStreamEvent> {
+	const call = (n: string) => ({ type: "function_call", id: `fc_${n}`, call_id: `call_${n}`, name: "read_file" });
+	const events = [
+		{ type: "response.output_item.added", output_index: 0, item: { ...call("a"), arguments: "" } },
+		{ type: "response.output_item.added", output_index: 1, item: { ...call("b"), arguments: "" } },
+		{ type: "response.function_call_arguments.delta", output_index: 0, item_id: "fc_a", delta: '{"path":"/a"}' },
+		{ type: "response.function_call_arguments.delta", output_index: 1, item_id: "fc_b", delta: '{"path":"/b"}' },
+		{
+			type: "response.output_item.added",
+			output_index: 0,
+			item: { type: "message", id: "msg_reused", role: "assistant", status: "in_progress", content: [] },
+		},
+		{
+			type: "response.output_item.done",
+			output_index: 0,
+			item: { type: "message", id: "msg_reused", role: "assistant", status: "completed", content: [] },
+		},
+		{ type: "response.output_item.done", output_index: 0, item: { ...call("a"), arguments: '{"path":"/a"}' } },
+		{ type: "response.output_item.done", output_index: 1, item: { ...call("b"), arguments: '{"path":"/b"}' } },
+		{ type: "response.completed", response: { id: "resp_reused_index", status: "completed" } },
+	];
+	for (const event of events) yield event as unknown as ResponseStreamEvent;
+}
+
 describe("OpenAI Responses terminal event handling", () => {
 	it("rejects streams that end before a terminal response event", async () => {
 		const model = createModel();
@@ -214,6 +278,36 @@ describe("OpenAI Responses terminal event handling", () => {
 		);
 	});
 
+	it("rejects completed streams whose tool call never received output_item.done", async () => {
+		const model = createModel();
+		const output = createOutput(model);
+		const stream = new AssistantMessageEventStream();
+
+		await expect(processResponsesStream(createUnfinishedToolCallEvents(), output, stream, model)).rejects.toThrow(
+			"OpenAI Responses stream completed with an unfinished tool call: bash (call_1|fc_1)",
+		);
+	});
+
+	it("rejects parallel tool calls without output_index instead of running mixed-up calls", async () => {
+		const model = createModel();
+		const output = createOutput(model);
+		const stream = new AssistantMessageEventStream();
+
+		await expect(
+			processResponsesStream(createToolCallsWithoutOutputIndexEvents(), output, stream, model),
+		).rejects.toThrow("OpenAI Responses stream completed with an unfinished tool call: bash (call_a|fc_a)");
+	});
+
+	it("rejects tool calls rebuilt from a reused output_index", async () => {
+		const model = createModel();
+		const output = createOutput(model);
+		const stream = new AssistantMessageEventStream();
+
+		await expect(processResponsesStream(createReusedOutputIndexEvents(), output, stream, model)).rejects.toThrow(
+			"OpenAI Responses stream completed with an unfinished tool call: read_file (call_a|fc_a)",
+		);
+	});
+
 	it("emits an error final result when the wrapper stream ends before a terminal response event", async () => {
 		const model = createModel();
 		const context: Context = {