diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 1aa55ac65..a69ef0bd7 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -796,7 +796,8 @@ export const streamAnthropic: StreamFunction<"anthropic-messages"> = (
 				resolveAnthropicBaseUrl(model, options?.apiKey ?? getEnvApiKey(model.provider) ?? "") ??
 				"https://api.anthropic.com";
 			const providerSessionState = getAnthropicProviderSessionState(options?.providerSessionState);
-			let disableStrictTools = providerSessionState?.strictToolsDisabled ?? false;
+			let disableStrictTools =
+				(providerSessionState?.strictToolsDisabled ?? false) || (model.compat?.disableStrictTools ?? false);
 			let strictFallbackErrorMessage: string | undefined;
 			const prepareParams = async (): Promise<MessageCreateParamsStreaming> => {
 				let nextParams = buildParams(model, baseUrl, context, isOAuthToken, options, disableStrictTools);
@@ -1565,7 +1566,8 @@ function buildParams(
 		const effort =
 			options.effort ?? (requestedEffort ? mapEffortToAnthropicAdaptiveEffort(model, requestedEffort) : undefined);
 
-		if (mode === "anthropic-adaptive") {
+		const disableAdaptiveThinking = model.compat?.disableAdaptiveThinking ?? false;
+		if (mode === "anthropic-adaptive" && !disableAdaptiveThinking) {
 			// Starting with Claude Opus 4.7, adaptive thinking content is omitted from the
 			// response by default. Opt into summarized reasoning so thinking deltas keep
 			// streaming with human-readable content for callers that rely on it.
diff --git a/packages/ai/src/types.ts b/packages/ai/src/types.ts
index 8757f7101..4feed5ce2 100644
--- a/packages/ai/src/types.ts
+++ b/packages/ai/src/types.ts
@@ -567,6 +567,27 @@ export interface OpenAICompat {
 	toolStrictMode?: "all_strict" | "none";
 }
 
+/**
+ * Compatibility settings for anthropic-messages API.
+ * Use this to disable features that strict-by-default Anthropic accepts but
+ * that proxy gateways (Vertex AI, AWS Bedrock-style fronts, etc.) reject.
+ */
+export interface AnthropicCompat {
+	/**
+	 * Drop the top-level `strict: true` field on tool definitions. Vertex AI's
+	 * Anthropic-compatible endpoint rejects unknown tool fields with
+	 * `tools.<n>.custom.strict: Extra inputs are not permitted`.
+	 */
+	disableStrictTools?: boolean;
+	/**
+	 * Map adaptive thinking (`thinking: { type: "adaptive" }`) to
+	 * `{ type: "enabled", budget_tokens }`. Vertex AI rejects the `adaptive`
+	 * tag with `Input tag 'adaptive' ... does not match any of the expected
+	 * tags: 'disabled', 'enabled'`.
+	 */
+	disableAdaptiveThinking?: boolean;
+}
+
 /**
  * OpenRouter provider routing preferences.
  * Controls which upstream providers OpenRouter routes requests to.
@@ -619,8 +640,12 @@ export interface Model<TApi extends Api = any> {
 	priority?: number;
 	/** Canonical thinking capability metadata for this model. */
 	thinking?: ThinkingConfig;
-	/** Compatibility overrides for openai-completions API. If not set, auto-detected from baseUrl. */
-	compat?: TApi extends "openai-completions" ? OpenAICompat : never;
+	/** Compatibility overrides per API. If not set, auto-detected from baseUrl. */
+	compat?: TApi extends "openai-completions"
+		? OpenAICompat
+		: TApi extends "anthropic-messages"
+			? AnthropicCompat
+			: never;
 	/**
 	 * Which shape to use when exposing the Codex `apply_patch` tool to this model.
 	 * Generated catalog policy sets `"freeform"` for first-party GPT-5 Responses
diff --git a/packages/ai/test/issue-826-repro.test.ts b/packages/ai/test/issue-826-repro.test.ts
new file mode 100644
index 000000000..b9ad2adbb
--- /dev/null
+++ b/packages/ai/test/issue-826-repro.test.ts
@@ -0,0 +1,126 @@
+import { describe, expect, it } from "bun:test";
+import { streamAnthropic } from "../src/providers/anthropic";
+import type { Context, Model, Tool } from "../src/types";
+
+const baseModel: Model<"anthropic-messages"> = {
+	id: "claude-sonnet-4-5",
+	name: "Claude Sonnet 4.5",
+	api: "anthropic-messages",
+	provider: "anthropic",
+	baseUrl:
+		"https://us-east5-aiplatform.googleapis.com/v1/projects/example/locations/us-east5/publishers/anthropic/models/claude-sonnet-4-5:streamRawPredict",
+	reasoning: false,
+	input: ["text"],
+	cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+	contextWindow: 200_000,
+	maxTokens: 8_192,
+};
+
+const bashTool: Tool = {
+	name: "bash",
+	description: "run a bash command",
+	parameters: {
+		type: "object",
+		properties: { command: { type: "string" } },
+		required: ["command"],
+	} as unknown as Tool["parameters"],
+};
+
+const baseContext: Context = {
+	systemPrompt: "Stay concise.",
+	messages: [{ role: "user", content: "Hi", timestamp: Date.now() }],
+	tools: [bashTool],
+};
+
+function abortedSignal(): AbortSignal {
+	const controller = new AbortController();
+	controller.abort();
+	return controller.signal;
+}
+
+function captureParams(
+	model: Model<"anthropic-messages">,
+): Promise<{ tools?: Array<{ name: string; strict?: unknown }> }> {
+	const { promise, resolve } = Promise.withResolvers<{ tools?: Array<{ name: string; strict?: unknown }> }>();
+	void streamAnthropic(model, baseContext, {
+		apiKey: "sk-ant-api-test",
+		isOAuth: false,
+		signal: abortedSignal(),
+		onPayload: payload => {
+			resolve(payload as { tools?: Array<{ name: string; strict?: unknown }> });
+			return undefined;
+		},
+	});
+	return promise;
+}
+
+describe("issue #826: Anthropic strict-tools opt-out for Vertex-style proxies", () => {
+	it("preserves strict:true on allowlisted tools by default (api.anthropic.com baseline)", async () => {
+		const params = await captureParams(baseModel);
+		const bash = params.tools?.find(t => t.name === "bash");
+		expect(bash).toBeDefined();
+		expect(bash?.strict).toBe(true);
+	});
+
+	it("omits strict on tool defs when compat.disableStrictTools is set", async () => {
+		const params = await captureParams({
+			...baseModel,
+			compat: { disableStrictTools: true },
+		});
+		const bash = params.tools?.find(t => t.name === "bash");
+		expect(bash).toBeDefined();
+		expect(bash?.strict).toBeUndefined();
+	});
+
+	it("preserves adaptive thinking by default", async () => {
+		const adaptiveModel: Model<"anthropic-messages"> = {
+			...baseModel,
+			id: "claude-opus-4-7",
+			reasoning: true,
+			thinking: {
+				mode: "anthropic-adaptive",
+				supportsAdaptiveEffort: true,
+			} as Model<"anthropic-messages">["thinking"],
+		};
+		const { promise, resolve } = Promise.withResolvers<{ thinking?: { type?: string } }>();
+		void streamAnthropic(adaptiveModel, baseContext, {
+			apiKey: "sk-ant-api-test",
+			isOAuth: false,
+			signal: abortedSignal(),
+			thinkingEnabled: true,
+			onPayload: payload => {
+				resolve(payload as { thinking?: { type?: string } });
+				return undefined;
+			},
+		});
+		const params = await promise;
+		expect(params.thinking?.type).toBe("adaptive");
+	});
+
+	it("maps adaptive thinking to enabled when compat.disableAdaptiveThinking is set", async () => {
+		const adaptiveModel: Model<"anthropic-messages"> = {
+			...baseModel,
+			id: "claude-opus-4-7",
+			reasoning: true,
+			thinking: {
+				mode: "anthropic-adaptive",
+				supportsAdaptiveEffort: true,
+			} as Model<"anthropic-messages">["thinking"],
+			compat: { disableAdaptiveThinking: true },
+		};
+		const { promise, resolve } = Promise.withResolvers<{ thinking?: { type?: string; budget_tokens?: number } }>();
+		void streamAnthropic(adaptiveModel, baseContext, {
+			apiKey: "sk-ant-api-test",
+			isOAuth: false,
+			signal: abortedSignal(),
+			thinkingEnabled: true,
+			onPayload: payload => {
+				resolve(payload as { thinking?: { type?: string; budget_tokens?: number } });
+				return undefined;
+			},
+		});
+		const params = await promise;
+		expect(params.thinking?.type).toBe("enabled");
+		expect(typeof params.thinking?.budget_tokens).toBe("number");
+	});
+});