diff --git a/packages/ai/src/providers/openai-completions.ts b/packages/ai/src/providers/openai-completions.ts
index b0673bb65..ee924d5d5 100644
--- a/packages/ai/src/providers/openai-completions.ts
+++ b/packages/ai/src/providers/openai-completions.ts
@@ -1372,7 +1372,7 @@ function buildParams(
 			openRouterParams.reasoning = { enabled: false };
 		} else if (options?.reasoning) {
 			openRouterParams.reasoning = {
-				effort: mapReasoningEffort(options.reasoning, model.thinking?.effortMap),
+				effort: mapReasoningEffort(options.reasoning, resolveActiveEffortMap(model, compat)),
 			};
 		}
 	} else if (
@@ -1383,7 +1383,7 @@ function buildParams(
 		compat.supportsReasoningEffort
 	) {
 		// OpenAI-style reasoning_effort
-		params.reasoning_effort = mapReasoningEffort(options.reasoning, model.thinking?.effortMap) as Effort;
+		params.reasoning_effort = mapReasoningEffort(options.reasoning, resolveActiveEffortMap(model, compat)) as Effort;
 	} else if (
 		supportsReasoningParams &&
 		options?.disableReasoning &&
@@ -1398,7 +1398,7 @@ function buildParams(
 		if (minEffort === undefined) {
 			throw new Error(`Model ${model.provider}/${model.id} has no supported reasoning efforts`);
 		}
-		params.reasoning_effort = mapReasoningEffort(minEffort, model.thinking?.effortMap) as Effort;
+		params.reasoning_effort = mapReasoningEffort(minEffort, resolveActiveEffortMap(model, compat)) as Effort;
 	}
 
 	if (compat.disableReasoningOnToolChoice && params.tool_choice !== undefined) {
@@ -1541,6 +1541,23 @@ function mapReasoningEffort(
 	return reasoningEffortMap?.[effort] ?? effort;
 }
 
+/**
+ * Compose the effective effort-map for the current turn: catalog-baked
+ * `model.thinking.effortMap` is the default, with the active compat's
+ * `reasoningEffortMap` overlaid on top so `compat.whenThinking` variants
+ * (and custom raw `Model` configs) keep authoring their own overrides.
+ */
+function resolveActiveEffortMap(
+	model: Model<"openai-completions">,
+	compat: ResolvedOpenAICompat,
+): Partial<Record<NonNullable<OpenAICompletionsOptions["reasoning"]>, string>> | undefined {
+	const compatMap = compat.reasoningEffortMap;
+	const thinkingMap = model.thinking?.effortMap;
+	if (!compatMap || Object.keys(compatMap).length === 0) return thinkingMap;
+	if (!thinkingMap) return compatMap;
+	return { ...thinkingMap, ...compatMap };
+}
+
 function maybeAddAnthropicCacheControl(compat: ResolvedOpenAICompat, messages: ChatCompletionMessageParam[]): void {
 	if (compat.cacheControlFormat !== "anthropic") return;
 	// Anthropic-style caching requires cache_control on a text part. Add a breakpoint
diff --git a/packages/ai/src/providers/openai-responses.ts b/packages/ai/src/providers/openai-responses.ts
index f9f6a54b2..cc08cacb7 100644
--- a/packages/ai/src/providers/openai-responses.ts
+++ b/packages/ai/src/providers/openai-responses.ts
@@ -501,7 +501,10 @@ function buildParams(
 		options,
 		messages,
 		effort =>
-			mapReasoningEffort(effort as NonNullable<OpenAIResponsesOptions["reasoning"]>, model.thinking?.effortMap),
+			mapReasoningEffort(
+				effort as NonNullable<OpenAIResponsesOptions["reasoning"]>,
+				resolveActiveEffortMap(model),
+			),
 		options?.includeEncryptedReasoning ?? true,
 		options?.omitReasoningEffort ?? false,
 	);
@@ -520,6 +523,21 @@ function mapReasoningEffort(
 	return reasoningEffortMap?.[effort] ?? effort;
 }
 
+/**
+ * Compose the effective effort-map: catalog-baked `model.thinking.effortMap`
+ * is the default, with the resolved compat's `reasoningEffortMap` overlaid on
+ * top so custom raw `Model` configs keep authoring their own overrides.
+ */
+function resolveActiveEffortMap(
+	model: Model<"openai-responses">,
+): Partial<Record<NonNullable<OpenAIResponsesOptions["reasoning"]>, string>> | undefined {
+	const compatMap = model.compat.reasoningEffortMap;
+	const thinkingMap = model.thinking?.effortMap;
+	if (!compatMap || Object.keys(compatMap).length === 0) return thinkingMap;
+	if (!thinkingMap) return compatMap;
+	return { ...thinkingMap, ...compatMap };
+}
+
 function convertConversationMessages(
 	model: Model<"openai-responses">,
 	context: Context,
diff --git a/packages/ai/test/issue-2315-repro.test.ts b/packages/ai/test/issue-2315-repro.test.ts
index 172e4a652..47d3cbb46 100644
--- a/packages/ai/test/issue-2315-repro.test.ts
+++ b/packages/ai/test/issue-2315-repro.test.ts
@@ -104,6 +104,30 @@ describe("issue #2315 — MiniMax M2 / GPT-OSS catalog excludes unsupported reas
 		expect(body.reasoning_effort).toBe("low");
 	});
 
+	it("preserves a custom compat.whenThinking reasoningEffortMap override at request time", async () => {
+		const base = getBundledModel("fireworks", "minimax-m2.7") as Model<"openai-completions">;
+		// Custom proxy: minimal stays clamped to low, xhigh is force-mapped to high
+		// via a whenThinking variant — the swap was getting lost when this PR
+		// moved effort maps onto `model.thinking.effortMap`.
+		const model = buildModel({
+			id: base.id,
+			name: base.name,
+			api: "openai-completions",
+			provider: "fireworks",
+			baseUrl: base.baseUrl,
+			reasoning: true,
+			thinking: { mode: "effort", efforts: [Effort.Low, Effort.Medium, Effort.High] },
+			compat: { whenThinking: { reasoningEffortMap: { high: "max" } } },
+			input: base.input,
+			cost: base.cost,
+			contextWindow: base.contextWindow,
+			maxTokens: base.maxTokens,
+		});
+
+		const body = await capturePayload(model, { reasoning: Effort.High });
+		expect(body.reasoning_effort).toBe("max");
+	});
+
 	it("preserves low/medium/high passthrough on fireworks/minimax-m2.7", async () => {
 		const model = getBundledModel("fireworks", "minimax-m2.7") as Model<"openai-completions">;
 		const lowBody = await capturePayload(model, { reasoning: Effort.Low });