diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 8a8584db4..0d703fe36 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -5,6 +5,7 @@
 ### Fixed
 
 - Fixed OpenAI Responses replay treating a tool output as paired with a matching call that appeared later in the input, or a tool call as paired with an earlier output. Pair repair now respects wire order before preserving or synthesizing each side.
+- Fixed adaptive-thinking Anthropic models omitting the interleaved-thinking beta on signature-enforcing proxies, which caused persisted interleaved assistant turns to fail on replay ([#6717](https://github.com/can1357/oh-my-pi/issues/6717)).
 
 ## [17.1.4] - 2026-07-26
 
diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 869847735..dc08672d4 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -2,7 +2,8 @@ import * as nodeCrypto from "node:crypto";
 import * as fs from "node:fs";
 import { scheduler } from "node:timers/promises";
 import * as tls from "node:tls";
-import { isOfficialAnthropicApiUrl } from "@oh-my-pi/pi-catalog/compat/anthropic";
+import { isAnthropicSigningProxyUrl, isOfficialAnthropicApiUrl } from "@oh-my-pi/pi-catalog/compat/anthropic";
+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";
@@ -2850,9 +2851,26 @@ export function buildAnthropicClientOptions(args: AnthropicClientOptionsArgs): A
 	} = args;
 	const compat = model.compat;
 	const disableStrictTools = disableStrictToolsOverride ?? compat.disableStrictTools;
-	const needsInterleavedBeta = interleavedThinking && !model.thinking?.supportsDisplay;
-	const oauthToken = isOAuth ?? isAnthropicOAuthToken(apiKey);
 	const baseUrl = resolveAnthropicBaseUrl(model, apiKey);
+	// Adaptive models (`supportsDisplay`) get native interleaved thinking on the
+	// 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.
+	// 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 ?? "") &&
+				!hostMatchesUrl(baseUrl, "githubCopilot")));
+	const oauthToken = isOAuth ?? isAnthropicOAuthToken(apiKey);
 	const supportsEagerToolInputStreaming = resolveEagerToolInputStreamingSupport(model, baseUrl);
 	const needsFineGrainedToolStreamingBeta =
 		hasTools && isOfficialAnthropicApiUrl(baseUrl) && !supportsEagerToolInputStreaming;
diff --git a/packages/ai/test/anthropic-alignment.test.ts b/packages/ai/test/anthropic-alignment.test.ts
index cbe94a480..cf9eb9797 100644
--- a/packages/ai/test/anthropic-alignment.test.ts
+++ b/packages/ai/test/anthropic-alignment.test.ts
@@ -1804,6 +1804,73 @@ describe("Anthropic request fingerprint alignment", () => {
 		expect(modern.defaultHeaders["anthropic-beta"] ?? "").not.toContain("interleaved-thinking-2025-05-14");
 	});
 
+	it("uses the effective route for adaptive interleaved-thinking beta headers", () => {
+		const adaptiveProxySpec: ModelSpec<"anthropic-messages"> = {
+			...ANTHROPIC_MODEL_SPEC,
+			id: "claude-opus-4-8",
+			name: "Claude Opus 4.8",
+			provider: "custom-anthropic",
+			baseUrl: "https://proxy.example.com/anthropic",
+			thinking: {
+				mode: "anthropic-adaptive",
+				efforts: [Effort.Minimal, Effort.Low, Effort.Medium, Effort.High, Effort.XHigh],
+				supportsDisplay: true,
+			},
+		};
+		const signingProxyUrl = "https://gateway.ai.cloudflare.com/v1/account/gateway/anthropic";
+		const signingProxy = buildAnthropicClientOptions({
+			model: buildModel({ ...adaptiveProxySpec, baseUrl: signingProxyUrl }),
+			apiKey: "sk-proxy-test",
+			interleavedThinking: true,
+		});
+		const canonicalModel = buildModel({
+			...adaptiveProxySpec,
+			provider: "anthropic",
+			baseUrl: "https://api.anthropic.com",
+		});
+		const reroutedSigningProxy = buildAnthropicClientOptions({
+			// Runtime provider overrides replace baseUrl without rebuilding the
+			// canonical model's official-endpoint compat.
+			model: { ...canonicalModel, baseUrl: signingProxyUrl },
+			apiKey: "sk-proxy-test",
+			interleavedThinking: true,
+		});
+		const nonSigningProxy = buildAnthropicClientOptions({
+			model: buildModel(adaptiveProxySpec),
+			apiKey: "sk-proxy-test",
+			interleavedThinking: true,
+		});
+		// Vertex rawPredict is signing regardless of provider id, but only
+		// accepts betas in the JSON body (`anthropic_beta`) (#5614).
+		const vertexRawPredict = buildAnthropicClientOptions({
+			model: buildModel({
+				...adaptiveProxySpec,
+				provider: "custom-vertex",
+				baseUrl:
+					"https://us-east5-aiplatform.googleapis.com/v1/projects/p/locations/us-east5/publishers/anthropic/models/claude-opus-4-8:rawPredict",
+			}),
+			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", () => {
 		const incompatibleModel: Model<"anthropic-messages"> = buildModel({
 			...ANTHROPIC_MODEL_SPEC,
diff --git a/packages/catalog/src/compat/anthropic.ts b/packages/catalog/src/compat/anthropic.ts
index af3a5ac6c..f5e1be87f 100644
--- a/packages/catalog/src/compat/anthropic.ts
+++ b/packages/catalog/src/compat/anthropic.ts
@@ -4,7 +4,7 @@
  * defaults come from provider ids, strict host checks, and model-id
  * classification, with explicit spec overrides assigned on top.
  */
-import { modelMatchesHost } from "../hosts";
+import { hostMatchesUrl, modelMatchesHost } from "../hosts";
 import {
 	hasOpus47ApiRestrictions,
 	isAnthropicFableOrMythosModel,
@@ -96,6 +96,23 @@ function isAzureAnthropicRoute(baseUrl?: string): boolean {
 	return baseUrl !== undefined && AZURE_ANTHROPIC_URL_MARKER.test(baseUrl);
 }
 
+/**
+ * Known non-official URLs that enforce Anthropic thinking signatures on replay.
+ *
+ * Runtime routing calls this with the effective URL because a model's resolved
+ * compat can be stale after Foundry or a provider base-URL override reroutes it.
+ */
+export function isAnthropicSigningProxyUrl(baseUrl?: string): boolean {
+	return (
+		hostMatchesUrl(baseUrl, "githubCopilot") ||
+		hostMatchesUrl(baseUrl, "zenmux") ||
+		isCloudflareAnthropicGateway(baseUrl) ||
+		isVertexAnthropicRoute(baseUrl) ||
+		isBedrockAnthropicRoute(baseUrl) ||
+		isAzureAnthropicRoute(baseUrl)
+	);
+}
+
 /** Build the resolved anthropic-messages compat record for a model spec. */
 export function buildAnthropicCompat(spec: ModelSpec<"anthropic-messages">): ResolvedAnthropicCompat {
 	const baseUrl = spec.baseUrl;
@@ -113,11 +130,8 @@ export function buildAnthropicCompat(spec: ModelSpec<"anthropic-messages">): Res
 	// (issue #4192).
 	const isZenmux = modelMatchesHost(spec, "zenmux");
 	const requiresThinkingEnabled = modelMatchesHost(spec, "moonshotNative") && matchesKimiMandatoryThinkingModel(spec);
-	const isVertex = isVertexAnthropicRoute(baseUrl);
-	const isBedrock = isBedrockAnthropicRoute(baseUrl);
 	const isAzure = isAzureAnthropicRoute(baseUrl);
-	const signingEndpoint =
-		official || isCopilot || isZenmux || isCloudflareAnthropicGateway(baseUrl) || isVertex || isBedrock || isAzure;
+	const signingEndpoint = official || isCopilot || isZenmux || isAnthropicSigningProxyUrl(baseUrl);
 	const compat: ResolvedAnthropicCompat = {
 		officialEndpoint: official,
 		signingEndpoint,