Add sessionAffinity header support

Add a sessionAffinity option across Workers AI adapters/providers to route requests with the same key to the same backend replica via the x-session-affinity header for prefix-cache optimization. Implementation details:

- Extend WorkersAiAdapterConfig with an optional sessionAffinity string.
- Propagate sessionAffinity as x-session-affinity to binding.run() via createWorkersAiBindingFetch(extraHeaders), to REST requests via defaultHeaders, and to gateway mode via createGatewayFetch call.
- Merge sessionAffinity with user-provided extraHeaders in the WorkersAI provider so both headers are forwarded together.

Other changes:

- Add and update tests covering binding.fetch, adapter behavior, and REST/binding header merging.
- Update README docs for tanstack-ai and workers-ai-provider to document sessionAffinity usage.
- Add changeset files to trigger a patch release for the relevant packages and minor formatting updates to demos.json.

Author: threepointone

.changeset/session-affinity-header.md

@@ -1,5 +1,5 @@
 ---
-"workers-ai-provider": minor
+"workers-ai-provider": patch
 ---
 
 Add `sessionAffinity` setting to send `x-session-affinity` header for prefix-cache optimization. Also forward `extraHeaders` in the REST API path instead of discarding them.

.changeset/tanstack-session-affinity.md

@@ -0,0 +1,5 @@
+---
+"@cloudflare/tanstack-ai": patch
+---
+
+Add `sessionAffinity` option to `WorkersAiAdapterConfig` for prefix-cache optimization. Routes requests with the same key to the same backend replica via the `x-session-affinity` header. Supported across binding, REST, and gateway modes.

demos.json

@@ -1,112 +1,112 @@
 {
-  "demos": {
-    "./demos/agent-scheduler": {
-      "package_json_hash": "2fe8785345d56ff37e675baaa06380c1eed736ba"
-    },
-    "./demos/agent-task-manager": {
-      "package_json_hash": "f2245bb30e95d0785aa195635a0928fba5b621ae"
-    },
-    "./demos/agent-task-manager-human-in-the-loop": {
-      "package_json_hash": "d8856a52bd0bf9641b8d1ff98e06de650aece34a"
-    },
-    "./demos/evaluator-optimiser": {
-      "package_json_hash": "8c4e9a71c91d806dcbef586b50a594e650d8f090"
-    },
-    "./demos/image-generation": {
-      "package_json_hash": "697d55539ad024faa349faa3dcd3bcbdfacf37bc"
-    },
-    "./demos/mcp-client": {
-      "package_json_hash": "5129c9edfcdd03c8625615c85329d85138fb9773"
-    },
-    "./demos/mcp-server-bearer-auth": {
-      "package_json_hash": "8703a8f8992a06377cce9a139ce6709450b51b5c"
-    },
-    "./demos/mcp-slack-oauth": {
-      "package_json_hash": "3134658fb11397626329bc344eba48fd57b21d46"
-    },
-    "./demos/mcp-stytch-b2b-okr-manager": {
-      "package_json_hash": "48232b81779a5f5fb0253842b4243c7dad032c0c"
-    },
-    "./demos/mcp-stytch-consumer-todo-list": {
-      "package_json_hash": "f53fe23dcebec62f51f9a6e332d2c192b8598cf6"
-    },
-    "./demos/model-scraper": {
-      "package_json_hash": "5a20ad46b257699c313bdd7c0b520701d739ed12"
-    },
-    "./demos/orchestrator-workers": {
-      "package_json_hash": "e159d1ce03c17bf13239ee4ac76c0290a210bc38"
-    },
-    "./demos/parallelisation": {
-      "package_json_hash": "6dbc55c3277b3ea634776821e60642b3dd03d8c0"
-    },
-    "./demos/prompt-chaining": {
-      "package_json_hash": "510159b05545a2d7f9c8cb240def56649cd25989"
-    },
-    "./demos/remote-mcp-authkit": {
-      "package_json_hash": "d3a0122c45d27140db96df6859e191aa7d2f8ac1"
-    },
-    "./demos/remote-mcp-github-oauth": {
-      "package_json_hash": "c59a2ecc4937d54c658383c3d7fe95e7c123f5c1"
-    },
-    "./demos/remote-mcp-server": {
-      "package_json_hash": "6240672fd54010c3b03a8af553b420306e11bc78"
-    },
-    "./demos/routing": {
-      "package_json_hash": "5f547b98f4e9a6167a2913e3a6c61681312986dd"
-    },
-    "./demos/structured-output": {
-      "package_json_hash": "a66aacd49c57e74c0937bf4bea0986168086debb"
-    },
-    "./demos/structured-output-node": {
-      "package_json_hash": "f64cc27508f9dda6fbb3bf4192c031dcc671e64a"
-    },
-    "./demos/text-generation": {
-      "package_json_hash": "d52767521e285b05c3235eaf2c8cc0e47fdbf90d"
-    },
-    "./demos/text-generation-stream": {
-      "package_json_hash": "f8272f5b1f5f1c83c53395dfc76646cab18a32b7"
-    },
-    "./demos/tool-calling": {
-      "package_json_hash": "3a0b1d91022d706b96e7b429c1349116ba9373b5"
-    },
-    "./demos/tool-calling-stream": {
-      "package_json_hash": "7c92250cda46aaac7eb6aeea0255828781c4abcb"
-    },
-    "./demos/tool-calling-stream-traditional": {
-      "package_json_hash": "c610c334d5f53a6e399bddddf68098ca0dec96d7"
-    },
-    "./demos/ui-worker": {
-      "package_json_hash": "831702fff4771ce9ce7d93afe6824ec6fa316125"
-    },
-    "./demos/remote-mcp-cf-access": {
-      "package_json_hash": "1a09d449c88cfe3b989f352d18813385578b98ca"
-    },
-    "./demos/remote-mcp-authless": {
-      "package_json_hash": "ba9953ce57a26cb271144e67609ed98fd1c1110e"
-    },
-    "./demos/python-workers-mcp": {
-      "package_json_hash": "0e710d7b27bb34edba396dc2b3365db230c076cb"
-    },
-    "./demos/vision": {
-      "package_json_hash": "e53450d50753f0574995feef3b2f845045fc3dc3"
-    },
-    "./demos/remote-mcp-google-oauth": {
-      "package_json_hash": "21bdab2ebbbe336c5fe6fb032fde804373f1b489"
-    },
-    "./demos/remote-mcp-logto": {
-      "package_json_hash": "a98a0cb367641ff86d89a7127f5e2551d2a1532f"
-    },
-    "./demos/remote-mcp-server-descope-auth": {
-      "package_json_hash": "c5de845803aae734fa60185200d4bfa2e1d0fb23"
-    },
-    "./demos/remote-mcp-server-autorag": {
-      "package_json_hash": "2b4e9b35192362b3be2743370469ce3a627a72b0"
-    },
-    "./demos/use-mcp-inspector": {
-      "package_json_hash": "d1d084f1aa9a752ead5250e0a070f97a9114dcea"
-    },
-    "./demos/hello-world": {
-      "package_json_hash": "ab24a12893c001fe3416fadea2a8bf5e7e68392e"
-    }
-  }
-}
+	"demos": {
+		"./demos/agent-scheduler": {
+			"package_json_hash": "2fe8785345d56ff37e675baaa06380c1eed736ba"
+		},
+		"./demos/agent-task-manager": {
+			"package_json_hash": "f2245bb30e95d0785aa195635a0928fba5b621ae"
+		},
+		"./demos/agent-task-manager-human-in-the-loop": {
+			"package_json_hash": "d8856a52bd0bf9641b8d1ff98e06de650aece34a"
+		},
+		"./demos/evaluator-optimiser": {
+			"package_json_hash": "8c4e9a71c91d806dcbef586b50a594e650d8f090"
+		},
+		"./demos/image-generation": {
+			"package_json_hash": "697d55539ad024faa349faa3dcd3bcbdfacf37bc"
+		},
+		"./demos/mcp-client": {
+			"package_json_hash": "5129c9edfcdd03c8625615c85329d85138fb9773"
+		},
+		"./demos/mcp-server-bearer-auth": {
+			"package_json_hash": "8703a8f8992a06377cce9a139ce6709450b51b5c"
+		},
+		"./demos/mcp-slack-oauth": {
+			"package_json_hash": "3134658fb11397626329bc344eba48fd57b21d46"
+		},
+		"./demos/mcp-stytch-b2b-okr-manager": {
+			"package_json_hash": "48232b81779a5f5fb0253842b4243c7dad032c0c"
+		},
+		"./demos/mcp-stytch-consumer-todo-list": {
+			"package_json_hash": "f53fe23dcebec62f51f9a6e332d2c192b8598cf6"
+		},
+		"./demos/model-scraper": {
+			"package_json_hash": "5a20ad46b257699c313bdd7c0b520701d739ed12"
+		},
+		"./demos/orchestrator-workers": {
+			"package_json_hash": "e159d1ce03c17bf13239ee4ac76c0290a210bc38"
+		},
+		"./demos/parallelisation": {
+			"package_json_hash": "6dbc55c3277b3ea634776821e60642b3dd03d8c0"
+		},
+		"./demos/prompt-chaining": {
+			"package_json_hash": "510159b05545a2d7f9c8cb240def56649cd25989"
+		},
+		"./demos/remote-mcp-authkit": {
+			"package_json_hash": "d3a0122c45d27140db96df6859e191aa7d2f8ac1"
+		},
+		"./demos/remote-mcp-github-oauth": {
+			"package_json_hash": "c59a2ecc4937d54c658383c3d7fe95e7c123f5c1"
+		},
+		"./demos/remote-mcp-server": {
+			"package_json_hash": "6240672fd54010c3b03a8af553b420306e11bc78"
+		},
+		"./demos/routing": {
+			"package_json_hash": "5f547b98f4e9a6167a2913e3a6c61681312986dd"
+		},
+		"./demos/structured-output": {
+			"package_json_hash": "a66aacd49c57e74c0937bf4bea0986168086debb"
+		},
+		"./demos/structured-output-node": {
+			"package_json_hash": "f64cc27508f9dda6fbb3bf4192c031dcc671e64a"
+		},
+		"./demos/text-generation": {
+			"package_json_hash": "d52767521e285b05c3235eaf2c8cc0e47fdbf90d"
+		},
+		"./demos/text-generation-stream": {
+			"package_json_hash": "f8272f5b1f5f1c83c53395dfc76646cab18a32b7"
+		},
+		"./demos/tool-calling": {
+			"package_json_hash": "3a0b1d91022d706b96e7b429c1349116ba9373b5"
+		},
+		"./demos/tool-calling-stream": {
+			"package_json_hash": "7c92250cda46aaac7eb6aeea0255828781c4abcb"
+		},
+		"./demos/tool-calling-stream-traditional": {
+			"package_json_hash": "c610c334d5f53a6e399bddddf68098ca0dec96d7"
+		},
+		"./demos/ui-worker": {
+			"package_json_hash": "831702fff4771ce9ce7d93afe6824ec6fa316125"
+		},
+		"./demos/remote-mcp-cf-access": {
+			"package_json_hash": "1a09d449c88cfe3b989f352d18813385578b98ca"
+		},
+		"./demos/remote-mcp-authless": {
+			"package_json_hash": "ba9953ce57a26cb271144e67609ed98fd1c1110e"
+		},
+		"./demos/python-workers-mcp": {
+			"package_json_hash": "0e710d7b27bb34edba396dc2b3365db230c076cb"
+		},
+		"./demos/vision": {
+			"package_json_hash": "e53450d50753f0574995feef3b2f845045fc3dc3"
+		},
+		"./demos/remote-mcp-google-oauth": {
+			"package_json_hash": "21bdab2ebbbe336c5fe6fb032fde804373f1b489"
+		},
+		"./demos/remote-mcp-logto": {
+			"package_json_hash": "a98a0cb367641ff86d89a7127f5e2551d2a1532f"
+		},
+		"./demos/remote-mcp-server-descope-auth": {
+			"package_json_hash": "c5de845803aae734fa60185200d4bfa2e1d0fb23"
+		},
+		"./demos/remote-mcp-server-autorag": {
+			"package_json_hash": "2b4e9b35192362b3be2743370469ce3a627a72b0"
+		},
+		"./demos/use-mcp-inspector": {
+			"package_json_hash": "d1d084f1aa9a752ead5250e0a070f97a9114dcea"
+		},
+		"./demos/hello-world": {
+			"package_json_hash": "ab24a12893c001fe3416fadea2a8bf5e7e68392e"
+		}
+	}
+}

packages/tanstack-ai/README.md

@@ -275,6 +275,15 @@ Workers AI supports four configuration modes:
 
 Third-party providers (OpenAI, Anthropic, Gemini, Grok, OpenRouter) only support the gateway modes.
 
+All Workers AI config modes also accept `sessionAffinity` to route requests with the same key to the same backend replica for prefix-cache optimization:
+
+```typescript
+const adapter = createWorkersAiChat("@cf/meta/llama-3.3-70b-instruct-fp8-fast", {
+	binding: env.AI,
+	sessionAffinity: "my-unique-session-id",
+});
+```
+
 ## Links
 
 - [TanStack AI Documentation](https://tanstack.com/ai)

packages/tanstack-ai/src/adapters/workers-ai.ts

@@ -33,11 +33,18 @@ export type WorkersAiTextModel =
 function buildWorkersAiClient(config: WorkersAiAdapterConfig): OpenAI {
 	validateWorkersAiConfig(config);
 
+	const sessionHeaders: Record<string, string> | undefined = config.sessionAffinity
+		? { "x-session-affinity": config.sessionAffinity }
+		: undefined;
+
 	if (isDirectBindingConfig(config)) {
 		// Plain binding mode: shim translates OpenAI fetch calls to env.AI.run()
 		return new OpenAI({
 			apiKey: "unused",
-			fetch: createWorkersAiBindingFetch(config.binding),
+			fetch: createWorkersAiBindingFetch(
+				config.binding,
+				sessionHeaders ? { extraHeaders: sessionHeaders } : undefined,
+			),
 		});
 	}
 
@@ -46,13 +53,14 @@ function buildWorkersAiClient(config: WorkersAiAdapterConfig): OpenAI {
 		return new OpenAI({
 			baseURL: `https://api.cloudflare.com/client/v4/accounts/${config.accountId}/ai/v1`,
 			apiKey: config.apiKey,
+			defaultHeaders: sessionHeaders,
 		});
 	}
 
 	// Gateway mode (existing): use createGatewayFetch
 	const gatewayConfig = config as AiGatewayAdapterConfig;
 	return new OpenAI({
-		fetch: createGatewayFetch("workers-ai", gatewayConfig),
+		fetch: createGatewayFetch("workers-ai", gatewayConfig, sessionHeaders),
 		apiKey: gatewayConfig.apiKey ?? "unused",
 	});
 }
@@ -377,11 +385,8 @@ export class WorkersAiTextAdapter<TModel extends WorkersAiTextModel> extends Bas
 
 				// Reasoning content (used by models like QwQ, DeepSeek R1, Kimi K2.5)
 				// The OpenAI SDK doesn't type this field, but models send it as an extension.
-				const reasoningContent = ((delta as Record<string, unknown>)
-						.reasoning_content ??
-						(delta as Record<string, unknown>).reasoning) as
-					| string
-					| undefined;
+				const reasoningContent = ((delta as Record<string, unknown>).reasoning_content ??
+					(delta as Record<string, unknown>).reasoning) as string | undefined;
 				if (reasoningContent) {
 					// RUN_STARTED is already guaranteed by the guard above
 					if (!hasEmittedStepStarted) {

packages/tanstack-ai/src/utils/create-fetcher.ts

@@ -105,10 +105,17 @@ export interface WorkersAiDirectCredentialsConfig {
  * upstream provider), distinct from `cfApiKey` (used in the `cf-aig-authorization`
  * header for authenticated gateways).
  */
-export type WorkersAiAdapterConfig =
+export type WorkersAiAdapterConfig = (
 	| WorkersAiDirectBindingConfig
 	| WorkersAiDirectCredentialsConfig
-	| (AiGatewayAdapterConfig & { apiKey?: string });
+	| (AiGatewayAdapterConfig & { apiKey?: string })
+) & {
+	/**
+	 * Session affinity key for prefix-cache optimization.
+	 * Routes requests with the same key to the same backend replica.
+	 */
+	sessionAffinity?: string;
+};
 
 // ---------------------------------------------------------------------------
 // Config detection helpers
@@ -330,7 +337,10 @@ function sanitizeToolCallId(id: string): string {
  * request parameters are extracted from the JSON body, matching Workers AI's
  * `binding.run(model, inputs)` calling convention.
  */
-export function createWorkersAiBindingFetch(binding: WorkersAiBinding): typeof fetch {
+export function createWorkersAiBindingFetch(
+	binding: WorkersAiBinding,
+	options?: { extraHeaders?: Record<string, string> },
+): typeof fetch {
 	return async (_input, init) => {
 		if (!init?.body) {
 			return new Response("No body", { status: 400 });
@@ -359,7 +369,11 @@ export function createWorkersAiBindingFetch(binding: WorkersAiBinding): typeof f
 		if (body.response_format) inputs.response_format = body.response_format;
 		if (stream) inputs.stream = true;
 
-		const result = await binding.run(model, inputs);
+		const result = await binding.run(
+			model,
+			inputs,
+			options?.extraHeaders ? { extraHeaders: options.extraHeaders } : undefined,
+		);
 
 		if (stream && result instanceof ReadableStream) {
 			// Workers AI returns an SSE stream with `data: {"response":"chunk"}` format.

packages/tanstack-ai/test/binding-fetch.test.ts

@@ -545,6 +545,44 @@ describe("createWorkersAiBindingFetch", () => {
 		expect(json.choices[0]!.finish_reason).toBe("stop");
 	});
 
+	it("should forward extraHeaders to binding.run() when configured", async () => {
+		const binding = mockBinding(vi.fn().mockResolvedValue({ response: "ok" }));
+
+		const fetcher = createWorkersAiBindingFetch(binding, {
+			extraHeaders: { "x-session-affinity": "session-123" },
+		});
+
+		await fetcher("https://api.openai.com/v1/chat/completions", {
+			method: "POST",
+			body: JSON.stringify({
+				model: "@cf/meta/llama-3.3-70b-instruct-fp8-fast",
+				messages: [{ role: "user", content: "Hi" }],
+			}),
+		});
+
+		expect(binding.run).toHaveBeenCalledOnce();
+		const [, , options] = binding.run.mock.calls[0]!;
+		expect(options).toEqual({ extraHeaders: { "x-session-affinity": "session-123" } });
+	});
+
+	it("should not pass extraHeaders to binding.run() when not configured", async () => {
+		const binding = mockBinding(vi.fn().mockResolvedValue({ response: "ok" }));
+
+		const fetcher = createWorkersAiBindingFetch(binding);
+
+		await fetcher("https://api.openai.com/v1/chat/completions", {
+			method: "POST",
+			body: JSON.stringify({
+				model: "@cf/meta/llama-3.3-70b-instruct-fp8-fast",
+				messages: [{ role: "user", content: "Hi" }],
+			}),
+		});
+
+		expect(binding.run).toHaveBeenCalledOnce();
+		const [, , options] = binding.run.mock.calls[0]!;
+		expect(options).toBeUndefined();
+	});
+
 	it("should pass response_format to binding for structured output", async () => {
 		const binding = mockBinding(vi.fn().mockResolvedValue({ response: '{"name":"test"}' }));