diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 8bdd61662..2af158fe7 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -1870,15 +1870,17 @@ const streamAnthropicOnce = (
 				// `context_management.clear_thinking_20251015` requires this beta. OAuth
 				// requests carry it in `claudeCodeAgentBetaDefaults`; API-key requests
 				// need it added explicitly so the field is honored instead of rejected
-				// (#3288). Skip transports where this package cannot deliver the beta
-				// in the form their adapter accepts: Copilot strips Anthropic betas,
-				// and Vertex rawPredict needs betas in the body (`anthropic_beta`),
-				// not as an `anthropic-beta` HTTP header.
+				// (#3288). Skip transports where this package cannot deliver or the
+				// provider cannot accept the beta: Copilot strips Anthropic betas;
+				// Vertex rawPredict needs betas in the body (`anthropic_beta`), not as
+				// an `anthropic-beta` HTTP header; and OpenCode Zen rejects the related
+				// `context_management` field (#6510).
 				if (
 					model.reasoning &&
 					options?.thinkingEnabled &&
 					model.provider !== "github-copilot" &&
 					model.provider !== "google-vertex" &&
+					model.provider !== "opencode-zen" &&
 					!extraBetas.includes(contextManagementBeta)
 				) {
 					extraBetas.push(contextManagementBeta);
diff --git a/packages/ai/test/issue-6510-repro.test.ts b/packages/ai/test/issue-6510-repro.test.ts
index 1385b9e33..e56921119 100644
--- a/packages/ai/test/issue-6510-repro.test.ts
+++ b/packages/ai/test/issue-6510-repro.test.ts
@@ -47,18 +47,26 @@ describe("issue #6510 — opencode-zen Anthropic auth + context_management", ()
 		expect(options.defaultHeaders.Authorization).toBeUndefined();
 	});
 
-	it("omits context_management on Zen thinking requests", async () => {
+	it("omits context_management and its beta on Zen thinking requests", async () => {
+		let capturedBeta: string | null = null;
+		const fetchMock = (async (_input: string | URL | Request, init?: RequestInit) => {
+			capturedBeta = new Headers(init?.headers).get("anthropic-beta");
+			return new Response(
+				JSON.stringify({ type: "error", error: { type: "invalid_request_error", message: "captured" } }),
+				{ status: 400, headers: { "Content-Type": "application/json" } },
+			);
+		}) as typeof fetch;
 		const { promise, resolve } = Promise.withResolvers<unknown>();
-		streamAnthropic(
+		await streamAnthropic(
 			ZEN_ANTHROPIC_MODEL,
 			{ systemPrompt: [], messages: [{ role: "user", content: "continue", timestamp: 0 }] },
 			{
 				apiKey: "sk-zen-test",
-				signal: AbortSignal.abort(),
 				thinkingEnabled: true,
+				fetch: fetchMock,
 				onPayload: payload => resolve(payload),
 			},
-		);
+		).result();
 
 		const payload = (await promise) as {
 			thinking?: { type?: string };
@@ -66,5 +74,6 @@ describe("issue #6510 — opencode-zen Anthropic auth + context_management", ()
 		};
 		expect(payload.thinking?.type).toBe("enabled");
 		expect(payload.context_management).toBeUndefined();
+		expect(capturedBeta ?? "").not.toContain("context-management-2025-06-27");
 	});
 });