diff --git a/extensions/copilot/package.json b/extensions/copilot/package.json
index 2c3bb392eeb..3bc9fcfb8f7 100644
--- a/extensions/copilot/package.json
+++ b/extensions/copilot/package.json
@@ -1967,9 +1967,10 @@
                     "type": "string",
                     "enum": [
                       "chat-completions",
-                      "responses"
+                      "responses",
+                      "messages"
                     ],
-                    "markdownDescription": "Body shape used to forward the reasoning effort to the model. `chat-completions` sends a top-level `reasoning_effort` string. `responses` sends a nested `reasoning.effort` object. When unset the format follows the URL: `/responses` → nested, otherwise top-level."
+                    "markdownDescription": "Body shape used to forward the reasoning effort to the model. `chat-completions` sends a top-level `reasoning_effort` string. `responses` sends a nested `reasoning.effort` object. `messages` sends the Anthropic Messages `output_config.effort` field. When unset the format follows the URL: `/responses` → nested, `/messages` → `output_config.effort`, otherwise top-level."
                   },
                   "requestHeaders": {
                     "type": "object",
@@ -2153,9 +2154,10 @@
                     "type": "string",
                     "enum": [
                       "chat-completions",
-                      "responses"
+                      "responses",
+                      "messages"
                     ],
-                    "markdownDescription": "Body shape used to forward the reasoning effort to the model. `chat-completions` sends a top-level `reasoning_effort` string. `responses` sends a nested `reasoning.effort` object. When unset the format follows the URL: `/responses` → nested, otherwise top-level."
+                    "markdownDescription": "Body shape used to forward the reasoning effort to the model. `chat-completions` sends a top-level `reasoning_effort` string. `responses` sends a nested `reasoning.effort` object. `messages` sends the Anthropic Messages `output_config.effort` field. When unset the format follows the URL: `/responses` → nested, `/messages` → `output_config.effort`, otherwise top-level."
                   },
                   "requestHeaders": {
                     "type": "object",
@@ -2306,9 +2308,10 @@
                     "type": "string",
                     "enum": [
                       "chat-completions",
-                      "responses"
+                      "responses",
+                      "messages"
                     ],
-                    "markdownDescription": "Body shape used to forward the reasoning effort to the model. `chat-completions` sends a top-level `reasoning_effort` string. `responses` sends a nested `reasoning.effort` object. When unset the format follows the URL: `/responses` → nested, otherwise top-level."
+                    "markdownDescription": "Body shape used to forward the reasoning effort to the model. `chat-completions` sends a top-level `reasoning_effort` string. `responses` sends a nested `reasoning.effort` object. `messages` sends the Anthropic Messages `output_config.effort` field. When unset the format follows the URL: `/responses` → nested, `/messages` → `output_config.effort`, otherwise top-level."
                   },
                   "requestHeaders": {
                     "type": "object",
diff --git a/extensions/copilot/src/extension/byok/common/byokProvider.ts b/extensions/copilot/src/extension/byok/common/byokProvider.ts
index a47b6d00637..adbfe439046 100644
--- a/extensions/copilot/src/extension/byok/common/byokProvider.ts
+++ b/extensions/copilot/src/extension/byok/common/byokProvider.ts
@@ -75,9 +75,10 @@ export interface BYOKModelCapabilities {
 	 * Override the body shape used to forward the reasoning effort to the model.
 	 * - `'chat-completions'`: top-level `reasoning_effort` (default for `/chat/completions`).
 	 * - `'responses'`: nested `reasoning.effort` (default for `/responses`).
-	 * If unset the format is inferred from whether the endpoint uses the Responses API.
+	 * - `'messages'`: `output_config.effort` (default for `/messages`).
+	 * If unset the format is inferred from the API path the endpoint uses.
 	 */
-	reasoningEffortFormat?: 'chat-completions' | 'responses';
+	reasoningEffortFormat?: 'chat-completions' | 'responses' | 'messages';
 }
 
 export interface BYOKModelRegistry {
diff --git a/extensions/copilot/src/extension/byok/node/openAIEndpoint.ts b/extensions/copilot/src/extension/byok/node/openAIEndpoint.ts
index aed5d1b64dd..ba73c73c9e3 100644
--- a/extensions/copilot/src/extension/byok/node/openAIEndpoint.ts
+++ b/extensions/copilot/src/extension/byok/node/openAIEndpoint.ts
@@ -268,7 +268,9 @@ export class OpenAIEndpoint extends ChatEndpoint {
 			return this._applyConfiguredModelOptions(body, options);
 		} else if (this.useMessagesApi) {
 			// Delegate to base ChatEndpoint for Messages API dispatch
-			return this._applyConfiguredModelOptions(super.createRequestBody(options), options);
+			const body = super.createRequestBody(options);
+			this._applyReasoningEffort(body, options);
+			return this._applyConfiguredModelOptions(body, options);
 		} else {
 			// Handle Chat Completions: provide callback for thinking data processing
 			const supportsThinking = !!this.modelMetadata.capabilities.supports.thinking;
@@ -320,7 +322,8 @@ export class OpenAIEndpoint extends ChatEndpoint {
 
 	/**
 	 * Forwards the per-request reasoning effort to the model body in the shape the endpoint expects.
-	 * Default shape mirrors the API path (`Responses` \u2192 nested `reasoning.effort`, `Chat Completions` \u2192 top-level `reasoning_effort`).
+	 * Default shape mirrors the API path (`Responses` \u2192 nested `reasoning.effort`, `Messages` \u2192 `output_config.effort`,
+	 * `Chat Completions` \u2192 top-level `reasoning_effort`).
 	 * `IChatModelInformation.reasoningEffortFormat` overrides the default so users hosting OpenAI-compatible servers
 	 * with diverging conventions (e.g. nested `reasoning.effort` on `/chat/completions`) can opt in deterministically.
 	 */
@@ -330,9 +333,9 @@ export class OpenAIEndpoint extends ChatEndpoint {
 			return;
 		}
 		const format = this.modelMetadata.reasoningEffortFormat
-			?? (this.useResponsesApi ? 'responses' : 'chat-completions');
+			?? (this.useResponsesApi ? 'responses' : this.useMessagesApi ? 'messages' : 'chat-completions');
 		const override = this._configurationService.getConfig(ConfigKey.Advanced.ReasoningEffortOverride);
-		const requested = override || options.modelCapabilities?.reasoningEffort || body.reasoning?.effort || body.reasoning_effort;
+		const requested = override || options.modelCapabilities?.reasoningEffort || body.reasoning?.effort || body.reasoning_effort || body.output_config?.effort;
 		const effort = requested && supports.includes(requested) ? requested : undefined;
 		// Scrub any pre-populated effort first so unsupported values (e.g. the hard-coded `medium` default
 		// from `createResponsesRequestBody`) cannot leak through, then write the resolved value into the
@@ -342,9 +345,16 @@ export class OpenAIEndpoint extends ChatEndpoint {
 			body.reasoning = Object.keys(rest).length > 0 ? rest : undefined;
 		}
 		body.reasoning_effort = undefined;
+		if (body.output_config) {
+			// Drop only the effort so other output_config fields (e.g. structured output format) survive
+			const { effort: _drop, ...rest } = body.output_config;
+			body.output_config = Object.keys(rest).length > 0 ? rest : undefined;
+		}
 		if (effort) {
 			if (format === 'responses') {
 				body.reasoning = { ...body.reasoning, effort };
+			} else if (format === 'messages') {
+				body.output_config = { ...body.output_config, effort };
 			} else {
 				body.reasoning_effort = effort;
 			}
diff --git a/extensions/copilot/src/extension/byok/node/test/openAIEndpoint.spec.ts b/extensions/copilot/src/extension/byok/node/test/openAIEndpoint.spec.ts
index e8716894eff..44bca53a257 100644
--- a/extensions/copilot/src/extension/byok/node/test/openAIEndpoint.spec.ts
+++ b/extensions/copilot/src/extension/byok/node/test/openAIEndpoint.spec.ts
@@ -527,6 +527,48 @@ describe('OpenAIEndpoint - Reasoning Properties', () => {
 			expect(body.reasoning?.effort).toBeUndefined();
 		});
 
+		it('places `output_config.effort` on the Messages API path when the model supports the requested level', () => {
+			const endpoint = instaService.createInstance(OpenAIEndpoint,
+				buildModel({ supported_endpoints: [ModelSupportedEndpoint.Messages] }),
+				'test-api-key',
+				'https://api.anthropic.com/v1/messages');
+
+			const body = endpoint.createRequestBody(buildOptions('high'));
+
+			expect(body.output_config).toEqual({ effort: 'high' });
+			expect(body.reasoning_effort).toBeUndefined();
+			expect(body.reasoning).toBeUndefined();
+		});
+
+		it('emits top-level `reasoning_effort` instead of `output_config.effort` when `reasoningEffortFormat` is `chat-completions` on a Messages URL', () => {
+			const endpoint = instaService.createInstance(OpenAIEndpoint,
+				buildModel({
+					reasoningEffortFormat: 'chat-completions',
+					supported_endpoints: [ModelSupportedEndpoint.Messages],
+				}),
+				'test-api-key',
+				'https://api.anthropic.com/v1/messages');
+
+			const body = endpoint.createRequestBody(buildOptions('high'));
+
+			expect(body.reasoning_effort).toBe('high');
+			expect(body.output_config).toBeUndefined();
+		});
+
+		it('scrubs `output_config.effort` while preserving other `output_config` fields', () => {
+			// `output_config` also carries structured-output fields (e.g. `format`); only the effort may be rewritten
+			const endpoint = instaService.createInstance(OpenAIEndpoint,
+				buildModel({ supported_endpoints: [ModelSupportedEndpoint.Messages] }),
+				'test-api-key',
+				'https://api.anthropic.com/v1/messages');
+			const apply = (endpoint as unknown as { _applyReasoningEffort: (body: IEndpointBody, options: ICreateEndpointBodyOptions) => void })._applyReasoningEffort.bind(endpoint);
+
+			const body: IEndpointBody = { output_config: { effort: 'unsupported-level', format: { type: 'json_schema', schema: {} } } as IEndpointBody['output_config'] };
+			apply(body, buildOptions('high'));
+
+			expect(body.output_config).toEqual({ format: { type: 'json_schema', schema: {} }, effort: 'high' });
+		});
+
 		it('does not emit a reasoning field when the model declares no reasoning support', () => {
 			const endpoint = instaService.createInstance(OpenAIEndpoint,
 				{
diff --git a/extensions/copilot/src/extension/byok/vscode-node/customEndpointProvider.ts b/extensions/copilot/src/extension/byok/vscode-node/customEndpointProvider.ts
index 565e6c05a8a..6bf6c10d170 100644
--- a/extensions/copilot/src/extension/byok/vscode-node/customEndpointProvider.ts
+++ b/extensions/copilot/src/extension/byok/vscode-node/customEndpointProvider.ts
@@ -104,7 +104,7 @@ interface _CustomEndpointModelConfig {
 	modelOptions?: IChatModelRequestOptions;
 	zeroDataRetentionEnabled?: boolean;
 	supportsReasoningEffort?: string[];
-	reasoningEffortFormat?: 'chat-completions' | 'responses';
+	reasoningEffortFormat?: 'chat-completions' | 'responses' | 'messages';
 }
 
 export interface CustomEndpointModelConfig extends _CustomEndpointModelConfig {
diff --git a/extensions/copilot/src/extension/byok/vscode-node/customOAIProvider.ts b/extensions/copilot/src/extension/byok/vscode-node/customOAIProvider.ts
index 318f14dedba..02da71ab718 100644
--- a/extensions/copilot/src/extension/byok/vscode-node/customOAIProvider.ts
+++ b/extensions/copilot/src/extension/byok/vscode-node/customOAIProvider.ts
@@ -66,7 +66,7 @@ interface _CustomOAIModelConfig {
 	requestHeaders?: Record<string, string>;
 	zeroDataRetentionEnabled?: boolean;
 	supportsReasoningEffort?: string[];
-	reasoningEffortFormat?: 'chat-completions' | 'responses';
+	reasoningEffortFormat?: 'chat-completions' | 'responses' | 'messages';
 }
 
 export interface CustomOAIModelConfig extends _CustomOAIModelConfig {
diff --git a/extensions/copilot/src/platform/endpoint/common/endpointProvider.ts b/extensions/copilot/src/platform/endpoint/common/endpointProvider.ts
index 28d7f2283de..d53f9bb744a 100644
--- a/extensions/copilot/src/platform/endpoint/common/endpointProvider.ts
+++ b/extensions/copilot/src/platform/endpoint/common/endpointProvider.ts
@@ -138,9 +138,9 @@ export type IChatModelInformation = IModelAPIResponse & {
 	/**
 	 * BYOK-only override that forces the body shape used when forwarding the reasoning effort to the model.
 	 * Honored by `OpenAIEndpoint`. Unset — the body shape follows the API path (Responses API → nested `reasoning.effort`,
-	 * Chat Completions → top-level `reasoning_effort`).
+	 * Anthropic Messages API → `output_config.effort`, Chat Completions → top-level `reasoning_effort`).
 	 */
-	reasoningEffortFormat?: 'chat-completions' | 'responses';
+	reasoningEffortFormat?: 'chat-completions' | 'responses' | 'messages';
 };
 
 export function isChatModelInformation(model: IModelAPIResponse): model is IChatModelInformation {