diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 3f4da9b40..15c9f504e 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed GitHub Copilot requests failing with a raw `HTTP 400 model_not_available_for_integrator` on roughly half of all turns for recently rolled-out models. Copilot's fleet is not uniform — part of it rejects models that `/models` advertises on the same host — and the transient classifier matched only the older `model_not_supported` code at a fixed envelope depth, so these rejections surfaced as terminal errors instead of entering the existing retry path. Model-availability 400s are now recognized at any envelope depth and rerolled on a flat delay with a dedicated 8-attempt budget on the OpenAI transports; every other retryable failure keeps its previous backoff and attempt count.
+
 ## [17.2.7] - 2026-08-03
 
 ### Changed
diff --git a/packages/ai/src/error/flags.ts b/packages/ai/src/error/flags.ts
index 09e54f17a..4e348493b 100644
--- a/packages/ai/src/error/flags.ts
+++ b/packages/ai/src/error/flags.ts
@@ -107,10 +107,17 @@ const STALE_RESPONSE_ITEM_DETAIL_PATTERN = /not[ _]?found|invalid|expired|stale|
 export const LLAMA_CPP_TOOL_CALL_PARSE_PATTERN =
 	/failed to parse tool call arguments as json|\[json\.exception\.parse_error\.101\]/i;
 
-// Copilot routing flap: HTTP 400 `model_not_supported` (structural code on the
-// error, also surfaced in text). Treated as transient — a retry usually lands
-// on a backend that has the model.
-const COPILOT_MODEL_NOT_SUPPORTED_PATTERN = /model_not_supported/i;
+// Copilot fleet skew: HTTP 400 rejecting a model that `/models` advertised on
+// the very same host. Two codes appear in the wild — `model_not_supported`
+// (per-OAuth-client rollout gap) and `model_not_available_for_integrator`
+// (replicas whose integrator allowlist predates the model). Both flap
+// request-to-request, so a retry usually lands on a backend that has the model.
+const COPILOT_TRANSIENT_MODEL_CODES: Record<string, true> = {
+	model_not_supported: true,
+	model_not_available_for_integrator: true,
+};
+const COPILOT_MODEL_UNAVAILABLE_PATTERN =
+	/model_not_supported|model_not_available_for_integrator|not available for integrator/i;
 // Anthropic strict-tool grammar too large / schema too complex (400 invalid_request_error).
 // Feature-gated deployments (Azure Foundry, Baseten, …) reject `strict: true`
 // tools outright when the hosted model lacks structured outputs, e.g.
@@ -345,8 +352,8 @@ function classifyText(errorMessage: string | undefined, errorStatus: number | un
 			kinds |= Flag.StaleResponsesItem;
 		}
 
-		// Copilot per-client routing flap is transient.
-		if (statusClean === 400 && COPILOT_MODEL_NOT_SUPPORTED_PATTERN.test(cleanMessage)) kinds |= Flag.Transient;
+		// Copilot fleet-skew model rejection is transient.
+		if (statusClean === 400 && COPILOT_MODEL_UNAVAILABLE_PATTERN.test(cleanMessage)) kinds |= Flag.Transient;
 		if (matchesStrictToolsRejection(cleanMessage, statusClean)) kinds |= Flag.Grammar;
 		if (matchesFastModeUnsupported(cleanMessage, statusClean)) kinds |= Flag.FastModeUnsupported;
 	}
@@ -460,16 +467,37 @@ export function isFastModeUnsupported(error: unknown): boolean {
 }
 
 /**
- * GitHub Copilot 400 `model_not_supported` routing flap — transient. Reads the
- * structural `code` (and falls back to {@link Flag.Transient} text classification).
+ * Depth-bounded search for a provider error `code`. SDK error objects keep the
+ * parsed response body on `.error`, and Copilot's body is itself
+ * `{ error: { code } }`, so the code sits up to two envelopes below the thrown
+ * error depending on which SDK produced it.
+ */
+function providerErrorCode(error: object): string | undefined {
+	let node: object = error;
+	for (let depth = 0; depth < 3; depth++) {
+		if ("code" in node && typeof node.code === "string") return node.code;
+		if (!("error" in node)) return undefined;
+		const nested: unknown = node.error;
+		if (!nested || typeof nested !== "object") return undefined;
+		node = nested;
+	}
+	return undefined;
+}
+
+/**
+ * GitHub Copilot 400 rejecting a model its own `/models` catalog advertises —
+ * transient fleet skew, not a malformed request. Reads the structural `code`
+ * through the SDK/body envelopes, then falls back to the stringified body both
+ * SDK families put in `message` (shapes drift; the wire text does not).
  */
 export function isCopilotTransientModelError(error: unknown): boolean {
-	if (status(error) === 400 && error && typeof error === "object") {
-		const info = error as { code?: unknown; error?: { code?: unknown } | null };
-		const code = typeof info.code === "string" ? info.code : info.error?.code;
-		if (code === "model_not_supported") return true;
-	}
-	return false;
+	if (!error || typeof error !== "object" || status(error) !== 400) return false;
+	const code = providerErrorCode(error);
+	// `Object.hasOwn`, not a bare index: `code` is provider-controlled, and a
+	// prototype key (`__proto__`, `toString`, …) would otherwise read truthy.
+	if (code !== undefined && Object.hasOwn(COPILOT_TRANSIENT_MODEL_CODES, code)) return true;
+	const message: unknown = "message" in error ? error.message : undefined;
+	return typeof message === "string" && COPILOT_MODEL_UNAVAILABLE_PATTERN.test(message);
 }
 
 export function classifyMessage(message: {
diff --git a/packages/ai/src/error/retryable.ts b/packages/ai/src/error/retryable.ts
index 90a065ee3..af2f3ae87 100644
--- a/packages/ai/src/error/retryable.ts
+++ b/packages/ai/src/error/retryable.ts
@@ -33,7 +33,7 @@ function isTransientTransportMessage(message: string): boolean {
 export interface ProviderRetryableHooks {
 	/** Provider id of the failing request, used to gate provider-specific checks. */
 	provider?: string;
-	/** Provider-specific transient predicate (e.g. Copilot `model_not_supported`). */
+	/** Provider-specific transient predicate (e.g. Copilot model-availability 400s). */
 	isProviderTransient?: (error: Error) => boolean;
 }
 
diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 880a6b42e..9232364b4 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -1531,6 +1531,13 @@ async function* observeDecodedAnthropicSdkEvents(
 
 const PROVIDER_MAX_RETRIES = 10;
 
+/**
+ * Flat delay between attempts when Copilot 400s a model its own `/models`
+ * catalog advertises. Part of the fleet carries the model and part doesn't, so
+ * the retry is a reroll rather than a wait for capacity to free up.
+ */
+const COPILOT_MODEL_FLAP_RETRY_DELAY_MS = 400;
+
 /**
  * How long `ping` keepalives may keep extending the idle deadline without any
  * semantic stream progress, as a multiple of the idle timeout. Anthropic pings
@@ -1561,8 +1568,8 @@ function shouldIgnoreAnthropicPreambleEvent(eventType: unknown): boolean {
 /**
  * Whether an Anthropic (or Copilot-over-Anthropic) stream error should be
  * retried. The classification lives in {@link AIError.isProviderRetryableError};
- * this wrapper injects the Copilot-specific `model_not_supported` transient
- * check, which the error module must not import directly.
+ * this wrapper injects the Copilot-specific model-availability transient check,
+ * which the error module must not import directly.
  */
 export function isProviderRetryableError(error: unknown, provider?: string): boolean {
 	return AIError.isProviderRetryableError(error, {
@@ -2709,7 +2716,12 @@ const streamAnthropicOnce = (
 						throw streamFailure;
 					}
 					providerRetryAttempt++;
-					const backoffDelayMs = calculateAnthropicRetryDelayMs(providerRetryAttempt - 1);
+					// Copilot's model-availability 400 is a per-request replica reroll, not
+					// upstream backpressure — the exponential curve would just add dead
+					// time to a coin flip that the next attempt is as likely to win.
+					const backoffDelayMs = AIError.isCopilotTransientModelError(streamFailure)
+						? COPILOT_MODEL_FLAP_RETRY_DELAY_MS
+						: calculateAnthropicRetryDelayMs(providerRetryAttempt - 1);
 					// Honor the server's retry hint (`retry-after-ms`/`retry-after`) on
 					// 429/529-style failures: retrying sooner than the server asked is a
 					// guaranteed failure that just burns the retry budget.
diff --git a/packages/ai/src/utils/http-inspector.ts b/packages/ai/src/utils/http-inspector.ts
index bf4f4e047..b62e0f5fd 100644
--- a/packages/ai/src/utils/http-inspector.ts
+++ b/packages/ai/src/utils/http-inspector.ts
@@ -95,10 +95,11 @@ export async function finalizeErrorMessage(
  * Rewrite error message for GitHub Copilot request failures.
  * Must run AFTER finalizeErrorMessage since it replaces the message entirely.
  *
- * 400 `model_not_supported` = Copilot routing rollout gap for our OAuth client.
- *        A preview model (gpt-5.3-codex, gpt-5.4*, ...) flaps between 200 and
- *        400 because only some of Copilot's backends have the model. After the
- *        in-request retry exhausts, surface guidance rather than the raw error.
+ * 400 model-unavailable = Copilot fleet skew. A model that `/models` advertises
+ *        (claude-sonnet-4.6, claude-opus-4.6, gpt-5.4, gpt-5.3-codex, ...)
+ *        flaps between 200 and 400 because only part of Copilot's fleet has it
+ *        in the integrator allowlist. After the in-request retry exhausts,
+ *        surface guidance rather than the raw error.
  * 401 = token invalid/expired → credential removal is safe, prompt re-login.
  * 403 = token valid but access denied (plan, model policy, org restriction) →
  *       do NOT reuse the auth-failed string (which triggers credential removal).
@@ -113,7 +114,7 @@ export function rewriteCopilotError(errorMessage: string, error: unknown, provid
 		return `GitHub Copilot access denied (HTTP 403). Your account may not have access to this model or feature. Check your Copilot plan or model policy settings.`;
 	}
 	if (isCopilotTransientModelError(error)) {
-		return `GitHub Copilot rejected this model (HTTP 400 model_not_supported) after retries. This is a known intermittent rollout gap for preview models on OAuth clients other than VS Code. Try again in a few seconds, switch to a GA model (gpt-5-mini, gpt-5.2), or run this model from VS Code.`;
+		return `GitHub Copilot rejected this model (HTTP 400) after retries: only part of its fleet currently serves this model id, even though /models advertises it. Try again in a few seconds or switch to a model Copilot serves fleet-wide (claude-opus-4.7, claude-sonnet-4.5, gpt-4.1).`;
 	}
 	return errorMessage;
 }
diff --git a/packages/ai/src/utils/retry.ts b/packages/ai/src/utils/retry.ts
index d89ee81b1..fce896bce 100644
--- a/packages/ai/src/utils/retry.ts
+++ b/packages/ai/src/utils/retry.ts
@@ -7,14 +7,25 @@ import { getHeadersFromError, getRetryAfterMsFromHeaders } from "./retry-after";
 // home). Re-exported here so existing `../utils/retry` importers keep working.
 export { isCopilotTransientModelError };
 
-const COPILOT_MODEL_RETRY_MAX_ATTEMPTS = 3;
+// Copilot's model-availability flap is a per-request coin flip across fleet
+// replicas, not backpressure. Measured per-attempt rejection rates for models
+// mid-rollout reach ~70% (gpt-5.4, 2026-08-04), and a live 10-turn run needed 6
+// attempts on one turn — so a small budget just pushes the failure up to the
+// agent-level retry, which restarts the whole turn. Eight attempts on a flat
+// delay keep the residual near 5% at p=0.7 and under 1% at p=0.5, bounded at
+// ~2.8s of dead time in the pathological case.
+const COPILOT_MODEL_RETRY_MAX_ATTEMPTS = 8;
+// Transport blips and status-bearing failures keep the pre-flap budget: they are
+// not coin flips, so a longer ramp only delays surfacing a persistent fault.
+const COPILOT_GENERIC_RETRY_MAX_ATTEMPTS = 3;
 const COPILOT_MODEL_RETRY_BASE_DELAY_MS = 400;
 /** Longest server-requested backoff we are willing to sit out before giving up. */
 const COPILOT_RETRY_AFTER_MAX_WAIT_MS = 30_000;
 
 /**
- * Wrap an initial Copilot request so transient `model_not_supported` 400s are
- * retried a small number of times. No-op for non-Copilot providers.
+ * Wrap an initial Copilot request so transient model-availability 400s
+ * (`model_not_supported`, `model_not_available_for_integrator`) are retried a
+ * small number of times. No-op for non-Copilot providers.
  *
  * The callback **MUST** create a fresh in-flight request each invocation — a
  * once-consumed AsyncIterable cannot be re-iterated.
@@ -38,8 +49,16 @@ export async function callWithCopilotModelRetry<T>(
 			if (options.signal?.aborted) throw error;
 			const transientModelError = isCopilotTransientModelError(error);
 			if (!transientModelError && !isRetryableError(error)) throw error;
-			if (attempt === COPILOT_MODEL_RETRY_MAX_ATTEMPTS - 1) break;
-			let delayMs = retryBaseDelayMs * (attempt + 1);
+			// Budget is per failure kind, counted over attempts already spent: the
+			// eight-attempt allowance only covers the cheap model-availability reroll.
+			const maxAttempts = transientModelError
+				? COPILOT_MODEL_RETRY_MAX_ATTEMPTS
+				: COPILOT_GENERIC_RETRY_MAX_ATTEMPTS;
+			if (attempt >= maxAttempts - 1) break;
+			// Reroll the model flap on a flat delay: a ramp only adds dead time to a
+			// coin flip the next attempt is equally likely to win. Generic retryable
+			// failures (429/5xx/transport) keep the linear backoff below.
+			let delayMs = transientModelError ? retryBaseDelayMs : retryBaseDelayMs * (attempt + 1);
 			if (!transientModelError) {
 				const errorStatus = status(error);
 				if (errorStatus !== undefined) {
diff --git a/packages/ai/test/anthropic-retry.test.ts b/packages/ai/test/anthropic-retry.test.ts
index 9f20af608..8ab9a26e0 100644
--- a/packages/ai/test/anthropic-retry.test.ts
+++ b/packages/ai/test/anthropic-retry.test.ts
@@ -105,4 +105,22 @@ describe("isProviderRetryableError", () => {
 		expect(isProviderRetryableError(err, "anthropic")).toBe(false);
 		expect(isProviderRetryableError(err)).toBe(false);
 	});
+
+	it("retries Copilot's model_not_available_for_integrator 400 from the Anthropic messages proxy", () => {
+		// Shape thrown by @anthropic-ai/sdk against api.githubcopilot.com/v1/messages:
+		// the parsed body lands on `.error` and is itself `{ error: { code } }`.
+		const body = {
+			error: {
+				message:
+					'The requested model is not available for integrator "copilot-language-server". Available models: [gpt-4.1 claude-opus-4.7]. Verify the correct Copilot-Integration-Id header is being sent.',
+				code: "model_not_available_for_integrator",
+				param: "model",
+				type: "invalid_request_error",
+			},
+		};
+		const err = new Error(`400 ${JSON.stringify(body)}`);
+		Object.assign(err, { status: 400, error: body });
+		expect(isProviderRetryableError(err, "github-copilot")).toBe(true);
+		expect(isProviderRetryableError(err, "anthropic")).toBe(false);
+	});
 });
diff --git a/packages/ai/test/copilot-retry.test.ts b/packages/ai/test/copilot-retry.test.ts
index 79b79d85f..850d26506 100644
--- a/packages/ai/test/copilot-retry.test.ts
+++ b/packages/ai/test/copilot-retry.test.ts
@@ -1,14 +1,29 @@
-import { describe, expect, it } from "bun:test";
+import { afterEach, describe, expect, it, vi } from "bun:test";
+import { scheduler } from "node:timers/promises";
 import { callWithCopilotModelRetry, isCopilotTransientModelError } from "@oh-my-pi/pi-ai/utils/retry";
 import { isRetryableError } from "@oh-my-pi/pi-utils";
 
-type ErrorShape = { status: number; code?: string; error?: { code?: string; message?: string }; message: string };
+afterEach(() => {
+	vi.restoreAllMocks();
+});
 
-function copilotError({ status, code, error, message }: ErrorShape): Error {
+type ErrorShape = {
+	status: number;
+	code?: string;
+	error?: { code?: string; message?: string } | { error: { code?: string; message?: string } };
+	message: string;
+	headers?: Record<string, string>;
+};
+
+function copilotError({ status, code, error, message, headers }: ErrorShape): Error {
 	const err = new Error(message);
-	(err as unknown as ErrorShape).status = status;
-	if (code !== undefined) (err as unknown as ErrorShape).code = code;
-	if (error !== undefined) (err as unknown as ErrorShape).error = error;
+	// Single sanctioned assertion point: `Error` carries no provider fields, and
+	// every test reads them back through the real classifier.
+	const shaped = err as unknown as ErrorShape;
+	shaped.status = status;
+	if (code !== undefined) shaped.code = code;
+	if (error !== undefined) shaped.error = error;
+	if (headers !== undefined) shaped.headers = headers;
 	return err;
 }
 
@@ -31,6 +46,31 @@ describe("isCopilotTransientModelError", () => {
 		expect(isCopilotTransientModelError(err)).toBe(true);
 	});
 
+	it("matches 400 model_not_available_for_integrator nested two envelopes deep (Anthropic SDK shape)", () => {
+		// api.githubcopilot.com/v1/messages: the SDK stores the parsed body on
+		// `.error`, and that body is itself `{ error: { code } }`.
+		const err = copilotError({
+			status: 400,
+			error: {
+				error: {
+					code: "model_not_available_for_integrator",
+					message: 'The requested model is not available for integrator "copilot-language-server".',
+				},
+			},
+			message:
+				'400 {"error":{"message":"The requested model is not available for integrator \\"copilot-language-server\\". Available models: [gpt-4.1 claude-opus-4.7]","code":"model_not_available_for_integrator","param":"model","type":"invalid_request_error"}}',
+		});
+		expect(isCopilotTransientModelError(err)).toBe(true);
+	});
+
+	it("falls back to the stringified body when no envelope exposes a code", () => {
+		const err = copilotError({
+			status: 400,
+			message: '400 {"error":{"message":"The requested model is not available for integrator \\"x\\"."}}',
+		});
+		expect(isCopilotTransientModelError(err)).toBe(true);
+	});
+
 	it("does not match other 400 codes", () => {
 		const err = copilotError({
 			status: 400,
@@ -40,6 +80,13 @@ describe("isCopilotTransientModelError", () => {
 		expect(isCopilotTransientModelError(err)).toBe(false);
 	});
 
+	it("does not match 400 codes that collide with Object.prototype keys", () => {
+		for (const code of ["__proto__", "constructor", "toString", "hasOwnProperty"]) {
+			const err = copilotError({ status: 400, code, message: "bad request" });
+			expect(isCopilotTransientModelError(err)).toBe(false);
+		}
+	});
+
 	it("does not match 401/403/500 regardless of code", () => {
 		for (const status of [401, 403, 500]) {
 			const err = copilotError({
@@ -74,7 +121,7 @@ describe("callWithCopilotModelRetry", () => {
 		expect(calls).toBe(1);
 	});
 
-	it("retries up to 3 attempts for Copilot transient errors and eventually throws the last error", async () => {
+	it("retries up to 8 attempts for Copilot transient errors and eventually throws the last error", async () => {
 		let calls = 0;
 		const err = copilotError({ status: 400, code: "model_not_supported", message: "transient" });
 		await expect(
@@ -86,7 +133,7 @@ describe("callWithCopilotModelRetry", () => {
 				{ provider: "github-copilot", retryBaseDelayMs: 0 },
 			),
 		).rejects.toBe(err);
-		expect(calls).toBe(3);
+		expect(calls).toBe(8);
 	});
 
 	it("succeeds on the second attempt when the first is transient", async () => {
@@ -141,9 +188,7 @@ describe("callWithCopilotModelRetry", () => {
 			async () => {
 				calls += 1;
 				if (calls === 1) {
-					const err = copilotError({ status: 429, message: "rate limited" });
-					(err as unknown as { headers: Record<string, string> }).headers = { "retry-after": "0.01" };
-					throw err;
+					throw copilotError({ status: 429, message: "rate limited", headers: { "retry-after": "0.01" } });
 				}
 				return "ok" as const;
 			},
@@ -153,6 +198,21 @@ describe("callWithCopilotModelRetry", () => {
 		expect(calls).toBe(2);
 	});
 
+	it("does not stretch a persistent Retry-After 429 across the flap budget", async () => {
+		let calls = 0;
+		const err = copilotError({ status: 429, message: "rate limited", headers: { "retry-after": "0.01" } });
+		await expect(
+			callWithCopilotModelRetry(
+				async () => {
+					calls += 1;
+					throw err;
+				},
+				{ provider: "github-copilot", retryBaseDelayMs: 0 },
+			),
+		).rejects.toBe(err);
+		expect(calls).toBe(3);
+	});
+
 	it("still retries status-less transport blips with the linear backoff", async () => {
 		let calls = 0;
 		const result = await callWithCopilotModelRetry(
@@ -171,6 +231,62 @@ describe("callWithCopilotModelRetry", () => {
 		expect(calls).toBe(2);
 	});
 
+	it("caps persistent generic retryable failures at the pre-flap budget", async () => {
+		let calls = 0;
+		const err = new Error(
+			'HTTP2StreamReset fetching "https://api.example.com/x". For more information, pass `verbose: true` in the second argument to fetch()',
+		);
+		await expect(
+			callWithCopilotModelRetry(
+				async () => {
+					calls += 1;
+					throw err;
+				},
+				{ provider: "github-copilot", retryBaseDelayMs: 0 },
+			),
+		).rejects.toBe(err);
+		expect(calls).toBe(3);
+	});
+
+	it("keeps the flat delay and the larger budget scoped to model flaps", async () => {
+		const flatWaits: number[] = [];
+		const rampWaits: number[] = [];
+		const record = (into: number[]) => {
+			const spy = vi.spyOn(scheduler, "wait");
+			spy.mockImplementation(async (delay?: number) => {
+				into.push(delay ?? 0);
+			});
+			return spy;
+		};
+
+		record(flatWaits);
+		await expect(
+			callWithCopilotModelRetry(
+				async () => {
+					throw copilotError({ status: 400, code: "model_not_available_for_integrator", message: "flap" });
+				},
+				{ provider: "github-copilot", retryBaseDelayMs: 100 },
+			),
+		).rejects.toBeInstanceOf(Error);
+		vi.restoreAllMocks();
+
+		record(rampWaits);
+		await expect(
+			callWithCopilotModelRetry(
+				async () => {
+					throw new Error(
+						'HTTP2StreamReset fetching "https://api.example.com/x". For more information, pass `verbose: true` in the second argument to fetch()',
+					);
+				},
+				{ provider: "github-copilot", retryBaseDelayMs: 100 },
+			),
+		).rejects.toBeInstanceOf(Error);
+		vi.restoreAllMocks();
+
+		expect(flatWaits).toEqual([100, 100, 100, 100, 100, 100, 100]);
+		expect(rampWaits).toEqual([100, 200]);
+	});
+
 	it("stops retrying when the caller aborts during backoff", async () => {
 		const controller = new AbortController();
 		controller.abort();
diff --git a/packages/ai/test/github-copilot-anthropic-fleet-skew.test.ts b/packages/ai/test/github-copilot-anthropic-fleet-skew.test.ts
new file mode 100644
index 000000000..3146cfac5
--- /dev/null
+++ b/packages/ai/test/github-copilot-anthropic-fleet-skew.test.ts
@@ -0,0 +1,125 @@
+import { afterEach, describe, expect, it, vi } from "bun:test";
+import { streamAnthropic } from "@oh-my-pi/pi-ai/providers/anthropic";
+import type { Context, Model } from "@oh-my-pi/pi-ai/types";
+import { buildModel } from "@oh-my-pi/pi-catalog/build";
+import { OPENCODE_HEADERS } from "@oh-my-pi/pi-catalog/wire/github-copilot";
+
+afterEach(() => {
+	vi.restoreAllMocks();
+});
+
+function makeCopilotClaudeModel(): Model<"anthropic-messages"> {
+	return buildModel({
+		id: "claude-sonnet-4.6",
+		name: "Claude Sonnet 4.6",
+		api: "anthropic-messages",
+		provider: "github-copilot",
+		baseUrl: "https://api.githubcopilot.com",
+		headers: { ...OPENCODE_HEADERS },
+		reasoning: true,
+		input: ["text"],
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 128_000,
+		maxTokens: 16_000,
+	});
+}
+
+const testContext: Context = {
+	messages: [{ role: "user", content: "hello", timestamp: Date.now() }],
+};
+
+/**
+ * Verbatim body served by `api.githubcopilot.com/v1/messages` when the request
+ * lands on a fleet replica whose integrator allowlist predates the model, even
+ * though `/models` on the same host advertises it.
+ */
+const FLEET_SKEW_BODY = {
+	error: {
+		message:
+			'The requested model is not available for integrator "copilot-language-server". Available models: [gpt-4.1 claude-opus-4.7 claude-sonnet-4.5]. Verify the correct Copilot-Integration-Id header is being sent.',
+		code: "model_not_available_for_integrator",
+		param: "model",
+		type: "invalid_request_error",
+	},
+};
+
+const SSE_EVENTS = [
+	{
+		type: "message_start",
+		message: {
+			id: "msg_fleet",
+			type: "message",
+			role: "assistant",
+			model: "claude-sonnet-4.6",
+			content: [],
+			stop_reason: null,
+			stop_sequence: null,
+			usage: { input_tokens: 7, output_tokens: 0 },
+		},
+	},
+	{ type: "content_block_start", index: 0, content_block: { type: "text", text: "" } },
+	{ type: "content_block_delta", index: 0, delta: { type: "text_delta", text: "second try" } },
+	{ type: "content_block_stop", index: 0 },
+	{
+		type: "message_delta",
+		delta: { stop_reason: "end_turn", stop_sequence: null },
+		usage: { output_tokens: 3 },
+	},
+	{ type: "message_stop" },
+];
+
+function sseResponse(): Response {
+	const body = `${SSE_EVENTS.map(event => `event: ${event.type}\ndata: ${JSON.stringify(event)}\n`).join("\n")}\n`;
+	return new Response(body, { status: 200, headers: { "Content-Type": "text/event-stream" } });
+}
+
+describe("GitHub Copilot Anthropic fleet skew", () => {
+	it("retries the model-availability 400 and completes on the next replica", async () => {
+		let attempts = 0;
+		const fetchMock = vi.fn(async () => {
+			attempts += 1;
+			if (attempts === 1) {
+				return new Response(JSON.stringify(FLEET_SKEW_BODY), {
+					status: 400,
+					headers: { "Content-Type": "application/json" },
+				});
+			}
+			return sseResponse();
+		});
+
+		const result = await streamAnthropic(makeCopilotClaudeModel(), testContext, {
+			apiKey: "ghu_test_copilot_token",
+			fetch: fetchMock as unknown as typeof fetch,
+			providerRetryWait: async () => {},
+		}).result();
+
+		expect(attempts).toBe(2);
+		expect(result.stopReason).toBe("stop");
+		expect(result.errorMessage).toBeUndefined();
+		expect(result.content).toMatchObject([{ type: "text", text: "second try" }]);
+		// Billing is per user prompt, not per wire attempt: a turn that burned an
+		// extra gateway-rejected attempt must still report one premium request.
+		expect(result.usage.premiumRequests).toBe(1);
+	});
+
+	it("surfaces fleet-skew guidance once every retry lands on a stale replica", async () => {
+		const fetchMock = vi.fn(
+			async () =>
+				new Response(JSON.stringify(FLEET_SKEW_BODY), {
+					status: 400,
+					headers: { "Content-Type": "application/json" },
+				}),
+		);
+
+		const result = await streamAnthropic(makeCopilotClaudeModel(), testContext, {
+			apiKey: "ghu_test_copilot_token",
+			fetch: fetchMock as unknown as typeof fetch,
+			providerRetryWait: async () => {},
+		}).result();
+
+		expect(result.stopReason).toBe("error");
+		expect(result.errorMessage).toContain("only part of its fleet");
+		// Every attempt is a fresh wire request, not a replayed promise.
+		expect(fetchMock.mock.calls.length).toBeGreaterThan(1);
+	});
+});
diff --git a/packages/ai/test/github-copilot-error.test.ts b/packages/ai/test/github-copilot-error.test.ts
index e7fce6edb..badf9be60 100644
--- a/packages/ai/test/github-copilot-error.test.ts
+++ b/packages/ai/test/github-copilot-error.test.ts
@@ -33,14 +33,16 @@ describe("rewriteCopilotError", () => {
 		expect(result).not.toContain("/login github-copilot");
 	});
 
-	it("rewrites 400 model_not_supported with rollout-gap guidance", () => {
-		const err = new Error("400 The requested model is not supported.");
-		(err as unknown as { status: number; code: string }).status = 400;
-		(err as unknown as { status: number; code: string }).code = "model_not_supported";
-		const result = rewriteCopilotError("original", err, "github-copilot");
-		expect(result).toContain("HTTP 400 model_not_supported");
-		expect(result).toContain("rollout gap");
-		expect(result).not.toContain("authentication failed");
+	it("rewrites 400 model-unavailable codes with fleet-skew guidance", () => {
+		for (const code of ["model_not_supported", "model_not_available_for_integrator"]) {
+			const err = new Error("400 The requested model is not available.");
+			(err as unknown as { status: number; code: string }).status = 400;
+			(err as unknown as { status: number; code: string }).code = code;
+			const result = rewriteCopilotError("original", err, "github-copilot");
+			expect(result).toContain("HTTP 400");
+			expect(result).toContain("only part of its fleet");
+			expect(result).not.toContain("authentication failed");
+		}
 	});
 
 	it("leaves non-copilot 400 model_not_supported untouched", () => {