diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 1de0793f..5619050e 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -8,6 +8,7 @@
 
 ### Fixed
 
+- Fixed `openai-responses` compatibility by adding `compat.sendSessionIdHeader: false`, allowing strict OpenAI-compatible proxies to omit the underscore-containing `session_id` header while still sending other session-affinity headers ([#3579](https://github.com/badlogic/pi-mono/issues/3579))
 - Fixed `anthropic-messages` tool streaming compatibility by adding `compat.supportsEagerToolInputStreaming`, allowing Anthropic-compatible providers to omit per-tool `eager_input_streaming` and use the legacy fine-grained tool streaming beta header instead ([#3575](https://github.com/badlogic/pi-mono/issues/3575))
 - Fixed `supportsXhigh()` to recognize `openai-codex` `gpt-5.5`, preserving `xhigh` reasoning requests instead of clamping them to `high`.
 - Fixed `openai-completions` streamed tool-call assembly to coalesce deltas by stable tool index when OpenAI-compatible gateways mutate tool call IDs mid-stream, preventing malformed Kimi K2.6/OpenCode tool streams from splitting one call into multiple bogus tool calls ([#3576](https://github.com/badlogic/pi-mono/issues/3576))
diff --git a/packages/ai/src/providers/openai-responses.ts b/packages/ai/src/providers/openai-responses.ts
index b8a991b6..135f57ef 100644
--- a/packages/ai/src/providers/openai-responses.ts
+++ b/packages/ai/src/providers/openai-responses.ts
@@ -180,7 +180,9 @@ function createClient(
 	}
 
 	if (sessionId) {
-		headers.session_id = sessionId;
+		if (model.compat?.sendSessionIdHeader !== false) {
+			headers.session_id = sessionId;
+		}
 		headers["x-client-request-id"] = sessionId;
 	}
 
diff --git a/packages/ai/src/types.ts b/packages/ai/src/types.ts
index aa45ba96..8710bbd6 100644
--- a/packages/ai/src/types.ts
+++ b/packages/ai/src/types.ts
@@ -300,7 +300,8 @@ export interface OpenAICompletionsCompat {
 
 /** Compatibility settings for OpenAI Responses APIs. */
 export interface OpenAIResponsesCompat {
-	// Reserved for future use
+	/** Whether to send the OpenAI `session_id` cache-affinity header from `options.sessionId` when caching is enabled. Default: true. */
+	sendSessionIdHeader?: boolean;
 }
 
 /**
diff --git a/packages/ai/test/openai-completions-tool-result-images.test.ts b/packages/ai/test/openai-completions-tool-result-images.test.ts
index 50fce94e..4f912278 100644
--- a/packages/ai/test/openai-completions-tool-result-images.test.ts
+++ b/packages/ai/test/openai-completions-tool-result-images.test.ts
@@ -54,7 +54,7 @@ function buildToolResult(toolCallId: string, timestamp: number): ToolResultMessa
 
 describe("openai-completions convertMessages", () => {
 	it("batches tool-result images after consecutive tool results", () => {
-		const baseModel = getModel("openai", "gpt-4o-mini");
+		const { compat: _compat, ...baseModel } = getModel("openai", "gpt-4o-mini");
 		const model: Model<"openai-completions"> = {
 			...baseModel,
 			api: "openai-completions",
diff --git a/packages/ai/test/openai-responses-copilot-provider.test.ts b/packages/ai/test/openai-responses-copilot-provider.test.ts
index ef55603b..cf25103c 100644
--- a/packages/ai/test/openai-responses-copilot-provider.test.ts
+++ b/packages/ai/test/openai-responses-copilot-provider.test.ts
@@ -108,6 +108,18 @@ describe("openai-responses provider defaults", () => {
 		expect(captured).toEqual({ sessionId: "session-123", clientRequestId: "session-123" });
 	});
 
+	it("can omit the session_id header while preserving other cache-affinity headers", async () => {
+		const proxyModel: Model<"openai-responses"> = {
+			...getModel("openai", "gpt-5.4"),
+			provider: "opencode",
+			baseUrl: "https://proxy.example.com/v1",
+			compat: { sendSessionIdHeader: false },
+		};
+		const captured = await captureOpenAIResponseHeaders({ sessionId: "session-123" }, proxyModel);
+
+		expect(captured).toEqual({ sessionId: null, clientRequestId: "session-123" });
+	});
+
 	it("lets explicit headers override the default OpenAI cache-affinity headers", async () => {
 		const captured = await captureOpenAIResponseHeaders({
 			sessionId: "session-123",