diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 3b109a6a5..dc08672d4 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -3,7 +3,7 @@ import * as fs from "node:fs";
 import { scheduler } from "node:timers/promises";
 import * as tls from "node:tls";
 import { isAnthropicSigningProxyUrl, isOfficialAnthropicApiUrl } from "@oh-my-pi/pi-catalog/compat/anthropic";
-import { isVertexRawPredictUrl } from "@oh-my-pi/pi-catalog/hosts";
+import { hostMatchesUrl, isVertexRawPredictUrl } from "@oh-my-pi/pi-catalog/hosts";
 import { mapEffortToAnthropicAdaptiveEffort } from "@oh-my-pi/pi-catalog/model-thinking";
 import { calculateCost, getBundledModel } from "@oh-my-pi/pi-catalog/models";
 import { isAnthropicOAuthToken } from "@oh-my-pi/pi-catalog/utils";
@@ -2856,15 +2856,20 @@ export function buildAnthropicClientOptions(args: AnthropicClientOptionsArgs): A
 	// official API, so only known non-official signing routes need the beta
 	// (#6717). Classify the effective URL: Foundry and provider overrides can
 	// reroute a model without rebuilding its materialized compat.
-	// Vertex rawPredict is excluded like every other HTTP-beta path here: it can
-	// only accept betas in the JSON body (`anthropic_beta`), never as this
-	// `anthropic-beta` HTTP header, so advertising it there earns a 400 (#5614).
+	// Two signing routes still can't take the beta as this `anthropic-beta` HTTP
+	// header, so they're excluded: Vertex rawPredict accepts betas only in the
+	// JSON body (`anthropic_beta`) and 400s on the header (#5614), and GitHub
+	// Copilot rejects Anthropic betas outright — the `github-copilot` provider
+	// branch below strips them, but a custom provider id or a canonical model
+	// rerouted to `api.githubcopilot.com` / `copilot-api.*` reaches the generic
+	// header builder instead, so exclude those effective URLs here too.
 	const needsInterleavedBeta =
 		interleavedThinking &&
 		(!model.thinking?.supportsDisplay ||
 			(!isOfficialAnthropicApiUrl(baseUrl) &&
 				isAnthropicSigningProxyUrl(baseUrl) &&
-				!isVertexRawPredictUrl(baseUrl ?? "")));
+				!isVertexRawPredictUrl(baseUrl ?? "") &&
+				!hostMatchesUrl(baseUrl, "githubCopilot")));
 	const oauthToken = isOAuth ?? isAnthropicOAuthToken(apiKey);
 	const supportsEagerToolInputStreaming = resolveEagerToolInputStreamingSupport(model, baseUrl);
 	const needsFineGrainedToolStreamingBeta =
diff --git a/packages/ai/test/anthropic-alignment.test.ts b/packages/ai/test/anthropic-alignment.test.ts
index 7028654a6..cf9eb9797 100644
--- a/packages/ai/test/anthropic-alignment.test.ts
+++ b/packages/ai/test/anthropic-alignment.test.ts
@@ -1852,11 +1852,23 @@ describe("Anthropic request fingerprint alignment", () => {
 			apiKey: "vertex-adc",
 			interleavedThinking: true,
 		});
+		// A custom provider on a Copilot host is signing, but the proxy rejects
+		// Anthropic betas outright and this path bypasses the provider branch.
+		const copilotUrlProxy = buildAnthropicClientOptions({
+			model: buildModel({
+				...adaptiveProxySpec,
+				provider: "custom-copilot",
+				baseUrl: "https://api.githubcopilot.com",
+			}),
+			apiKey: "ghu_test",
+			interleavedThinking: true,
+		});
 
 		expect(signingProxy.defaultHeaders["anthropic-beta"]).toContain("interleaved-thinking-2025-05-14");
 		expect(reroutedSigningProxy.defaultHeaders["anthropic-beta"]).toContain("interleaved-thinking-2025-05-14");
 		expect(nonSigningProxy.defaultHeaders["anthropic-beta"] ?? "").not.toContain("interleaved-thinking-2025-05-14");
 		expect(vertexRawPredict.defaultHeaders["anthropic-beta"] ?? "").not.toContain("interleaved-thinking-2025-05-14");
+		expect(copilotUrlProxy.defaultHeaders["anthropic-beta"] ?? "").not.toContain("interleaved-thinking-2025-05-14");
 	});
 
 	it("adds legacy fine-grained tool-streaming beta only for tool requests on incompatible models", () => {