diff --git a/packages/catalog/src/identity/family.ts b/packages/catalog/src/identity/family.ts
index 3eec81b7e..38a92e6a9 100644
--- a/packages/catalog/src/identity/family.ts
+++ b/packages/catalog/src/identity/family.ts
@@ -261,6 +261,19 @@ export const modelFamilyToken = memo((modelId: string): string => {
 	return "";
 });
 
+/**
+ * True for Claude generations that support extended thinking: Sonnet/Opus 3.7+,
+ * every 4.x/5+ Opus/Sonnet, and the Fable/Mythos generation. Pre-thinking
+ * models (Claude 3.5 and older) are excluded so no thinking effort dial is
+ * fabricated for a model that rejects thinking parameters. Classifier-based, so
+ * dotted and dashed version forms both match; ids the classifier does not parse
+ * (e.g. Haiku, bare dated ids) return false.
+ */
+export const anthropicModelSupportsThinking = memo((modelId: string): boolean => {
+	const parsed = parseAnthropicModel(bareModelId(modelId));
+	return parsed !== null && semverGte(parsed.version, "3.7");
+});
+
 /**
  * Adaptive thinking `display` is supported starting with Claude Opus 4.7+,
  * Sonnet 5+, and the Claude Fable/Mythos 5 generation. Older adaptive-thinking
diff --git a/packages/catalog/src/provider-models/openai-compat.ts b/packages/catalog/src/provider-models/openai-compat.ts
index cb278fa2e..18b7bb22c 100644
--- a/packages/catalog/src/provider-models/openai-compat.ts
+++ b/packages/catalog/src/provider-models/openai-compat.ts
@@ -7,6 +7,7 @@ import {
 import { Effort, THINKING_EFFORTS } from "../effort";
 import { FIREWORKS_FAST_SUFFIX, toFireworksPublicModelId } from "../fireworks-model-id";
 import {
+	anthropicModelSupportsThinking,
 	isGlmVisionModelId,
 	isGrokReasoningEffortCapable,
 	isKimiK3ModelId,
@@ -4385,12 +4386,16 @@ export function githubCopilotModelManagerOptions(config?: GithubCopilotModelMana
 									maxTokens,
 									headers: { ...COPILOT_API_HEADERS },
 									// Copilot's `/models` advertises no reasoning bit, so a
-									// Claude model without a bundled reference would fall back
-									// to `reasoning: false` and lose its thinking dial. Every
-									// Claude on Copilot's anthropic-messages proxy reasons;
-									// mark it so `buildModel` derives the adaptive effort
-									// ladder from the id (e.g. a newly served claude-opus-5).
-									...(api === "anthropic-messages" ? { reasoning: true } : {}),
+									// thinking-capable Claude with no bundled reference would
+									// fall back to `reasoning: false` and lose its effort dial.
+									// Gate on the id classifier (not the transport alone) so a
+									// lagging enterprise catalog serving a pre-thinking Claude
+									// (<= 3.5) over the Messages proxy is not handed a fabricated
+									// dial it would reject; a modern reference-less model (e.g.
+									// claude-opus-5) is marked so `buildModel` derives the ladder.
+									...(api === "anthropic-messages" && anthropicModelSupportsThinking(defaults.id)
+										? { reasoning: true }
+										: {}),
 									...(api === "openai-completions"
 										? {
 												compat: {
diff --git a/packages/catalog/test/issue-6664-repro.test.ts b/packages/catalog/test/issue-6664-repro.test.ts
index 987cd3c02..442ec455e 100644
--- a/packages/catalog/test/issue-6664-repro.test.ts
+++ b/packages/catalog/test/issue-6664-repro.test.ts
@@ -94,4 +94,26 @@ describe("#6664 github-copilot reference-less Claude model", () => {
 			await fs.rm(tempDir, { recursive: true, force: true });
 		}
 	});
+
+	it("does not fabricate a thinking dial for a pre-thinking reference-less Claude", async () => {
+		// A lagging enterprise catalog could surface an old kind-first Claude on
+		// the Messages proxy. It must stay non-reasoning so no effort dial is
+		// offered for thinking parameters the model would reject.
+		const fetchMock = vi.fn(
+			async () =>
+				new Response(JSON.stringify({ data: [tieredEntry("claude-sonnet-3.5", "Claude Sonnet 3.5")] }), {
+					status: 200,
+					headers: { "Content-Type": "application/json" },
+				}),
+		);
+		const options = githubCopilotModelManagerOptions({ apiKey: "copilot-test-key", fetch: fetchMock });
+		const specs = (await options.fetchDynamicModels?.()) ?? [];
+
+		const base = specs.find(m => m.id === "claude-sonnet-3.5");
+		expect(base).toBeDefined();
+		expect(base?.api).toBe("anthropic-messages");
+		expect(base?.reasoning).toBe(false);
+		if (!base) throw new Error("missing base spec");
+		expect(buildModel(base).thinking).toBeUndefined();
+	});
 });