diff --git a/changelog.d/fixes/agnes-thinking-effort-tiers.md b/changelog.d/fixes/agnes-thinking-effort-tiers.md
new file mode 100644
index 0000000000..3fc652bc1b
--- /dev/null
+++ b/changelog.d/fixes/agnes-thinking-effort-tiers.md
@@ -0,0 +1 @@
+- fix(providers): **declare Agnes chat models' live `reasoning_effort` vocabulary so catalog/builder/sanitizer stop inventing aliases the API 400s.** 2.0/2.5 accept `none/low/medium/high/max`; 3.0 also accepts `minimal` and `xhigh`. `off`/`ultra` still clamp off the wire.
diff --git a/open-sse/config/providers/registry/agnes/index.ts b/open-sse/config/providers/registry/agnes/index.ts
index 8e3a5cbdf5..cd8bf89bcb 100644
--- a/open-sse/config/providers/registry/agnes/index.ts
+++ b/open-sse/config/providers/registry/agnes/index.ts
@@ -1,5 +1,22 @@
 import type { RegistryEntry } from "../../shared.ts";
 
+// Official Agnes chat contract from live /v1/chat/completions probes
+// (2026-09-14, apihub.agnes-ai.com). 2.0/2.5 accept none/low/medium/high/max
+// and 400 on xhigh/off/ultra/minimal. 3.0 additionally accepts minimal and
+// xhigh. HuggingFace's Agnes-3.0-Flash card lists four of these
+// (none/low/medium/high); live 3.0 also takes minimal and xhigh, so the
+// registry follows the live API rather than the shorter card.
+export const AGNES_FLASH_THINKING_EFFORTS = ["none", "low", "medium", "high", "max"] as const;
+export const AGNES_30_THINKING_EFFORTS = [
+  "none",
+  "minimal",
+  "low",
+  "medium",
+  "high",
+  "xhigh",
+  "max",
+] as const;
+
 export const agnesProvider: RegistryEntry = {
   id: "agnes",
   format: "openai",
@@ -15,6 +32,7 @@ export const agnesProvider: RegistryEntry = {
       contextLength: 262144,
       maxOutputTokens: 65536,
       supportsReasoning: true,
+      supportedThinkingEfforts: [...AGNES_FLASH_THINKING_EFFORTS],
       supportsVision: true,
       toolCalling: true,
     },
@@ -24,18 +42,20 @@ export const agnesProvider: RegistryEntry = {
       contextLength: 524288,
       maxOutputTokens: 65536,
       supportsReasoning: true,
+      supportedThinkingEfforts: [...AGNES_FLASH_THINKING_EFFORTS],
       supportsVision: true,
       toolCalling: true,
       interleavedField: "reasoning_content",
     },
+    // Wiki (2026-09-10): agnes-3.0-flash is 512k context / 65,536
+    // output, same window as 2.5-flash. Live /v1/models lists it.
     {
-      // Wiki (2026-09-10) lists agnes-3.0-flash at 512K context / 65,536 max
-      // output, same window as 2.5 Flash. Live GET /v1/models includes it.
       id: "agnes-3.0-flash",
       name: "Agnes 3.0 Flash",
       contextLength: 524288,
       maxOutputTokens: 65536,
       supportsReasoning: true,
+      supportedThinkingEfforts: [...AGNES_30_THINKING_EFFORTS],
       supportsVision: true,
       toolCalling: true,
       interleavedField: "reasoning_content",
diff --git a/tests/unit/agnes-provider.test.ts b/tests/unit/agnes-provider.test.ts
index f1353c6e1a..a8bc7a5d50 100644
--- a/tests/unit/agnes-provider.test.ts
+++ b/tests/unit/agnes-provider.test.ts
@@ -9,13 +9,18 @@ process.env.DATA_DIR = TEST_DATA_DIR;
 
 const { APIKEY_PROVIDERS } = await import("../../src/shared/constants/providers.ts");
 const { VIDEO_PROVIDER_IDS } = await import("../../src/shared/constants/providers.ts");
-const { REGISTRY: providerRegistry } = await import("../../open-sse/config/providerRegistry.ts");
+const { REGISTRY: providerRegistry, getRegistryModelThinkingEfforts } =
+  await import("../../open-sse/config/providerRegistry.ts");
 const { IMAGE_PROVIDERS, getAllImageModels } =
   await import("../../open-sse/config/imageRegistry.ts");
 const { VIDEO_PROVIDERS, getAllVideoModels } =
   await import("../../open-sse/config/videoRegistry.ts");
 const { FREE_MODEL_BUDGETS } = await import("../../open-sse/config/freeModelCatalog.ts");
 const { DefaultExecutor } = await import("../../open-sse/executors/default.ts");
+const { sanitizeReasoningEffortForProvider } =
+  await import("../../open-sse/executors/base/reasoningEffort.ts");
+const { getThinkingCapabilityFields } =
+  await import("../../src/app/api/v1/models/catalogHelpers.ts");
 const { handleImageGeneration } = await import("../../open-sse/handlers/imageGeneration.ts");
 const { handleVideoGeneration } = await import("../../open-sse/handlers/videoGeneration.ts");
 const { resolveChatCoreTargetFormat } =
@@ -87,6 +92,7 @@ test("agnes ships the current public chat models with the correct capabilities",
   assert.equal(flash20.contextLength, 262144);
   assert.equal(flash20.maxOutputTokens, 65536);
   assert.equal(flash20.supportsReasoning, true);
+  assert.deepEqual(flash20.supportedThinkingEfforts, ["none", "low", "medium", "high", "max"]);
   assert.equal(flash20.supportsVision, true);
   assert.equal(flash20.toolCalling, true);
 
@@ -94,26 +100,97 @@ test("agnes ships the current public chat models with the correct capabilities",
   assert.ok(flash25, "agnes-2.5-flash must be defined");
   assert.equal(flash25.contextLength, 524288);
   assert.equal(flash25.maxOutputTokens, 65536);
+  assert.equal(flash25.supportsReasoning, true);
+  assert.deepEqual(flash25.supportedThinkingEfforts, ["none", "low", "medium", "high", "max"]);
 
   const flash30 = entry.models.find((m) => m.id === "agnes-3.0-flash");
   assert.ok(flash30, "agnes-3.0-flash must be defined");
   assert.equal(flash30.contextLength, 524288);
   assert.equal(flash30.maxOutputTokens, 65536);
   assert.equal(flash30.supportsReasoning, true);
+  assert.deepEqual(flash30.supportedThinkingEfforts, [
+    "none",
+    "minimal",
+    "low",
+    "medium",
+    "high",
+    "xhigh",
+    "max",
+  ]);
   assert.equal(flash30.supportsVision, true);
   assert.equal(flash30.toolCalling, true);
   assert.equal(flash30.interleavedField, "reasoning_content");
 });
 
+test("agnes chat models advertise official thinking vocabulary", () => {
+  for (const id of ["agnes-2.0-flash", "agnes-2.5-flash"]) {
+    assert.deepEqual(getRegistryModelThinkingEfforts("agnes", id), [
+      "none",
+      "low",
+      "medium",
+      "high",
+      "max",
+    ]);
+  }
+  assert.deepEqual(getRegistryModelThinkingEfforts("agnes", "agnes-3.0-flash"), [
+    "none",
+    "minimal",
+    "low",
+    "medium",
+    "high",
+    "xhigh",
+    "max",
+  ]);
+});
+
+test("agnes catalog effort_tiers match declared vocabulary, not six-tier fallback", () => {
+  const efforts = getRegistryModelThinkingEfforts("agnes", "agnes-3.0-flash");
+  assert.ok(efforts && efforts.length > 0);
+  assert.deepEqual(
+    getThinkingCapabilityFields("agnes", "agnes-3.0-flash", true, efforts, !efforts.length),
+    {
+      thinking: true,
+      supportsThinking: true,
+      effort_tiers: ["none", "minimal", "low", "medium", "high", "xhigh", "max"],
+    }
+  );
+});
+
+test("agnes sanitizer keeps official tiers and clamps undocumented ones", () => {
+  const clamp = (model: string, effort: string) =>
+    (
+      sanitizeReasoningEffortForProvider({ reasoning_effort: effort }, "agnes", model) as {
+        reasoning_effort?: string;
+      }
+    ).reasoning_effort;
+
+  assert.equal(clamp("agnes-3.0-flash", "none"), "none");
+  assert.equal(clamp("agnes-3.0-flash", "minimal"), "minimal");
+  assert.equal(clamp("agnes-3.0-flash", "low"), "low");
+  assert.equal(clamp("agnes-3.0-flash", "high"), "high");
+  assert.equal(clamp("agnes-3.0-flash", "xhigh"), "xhigh");
+  assert.equal(clamp("agnes-3.0-flash", "max"), "max");
+  assert.equal(clamp("agnes-3.0-flash", "ultra"), "max");
+  assert.equal(clamp("agnes-3.0-flash", "off"), "none");
+
+  // 2.0/2.5 reject xhigh (HTTP 400); clamp up to the next accepted tier (max).
+  assert.equal(clamp("agnes-2.0-flash", "xhigh"), "max");
+  assert.equal(clamp("agnes-2.5-flash", "xhigh"), "max");
+  assert.equal(clamp("agnes-2.0-flash", "max"), "max");
+  assert.equal(clamp("agnes-2.0-flash", "off"), "none");
+  assert.equal(clamp("agnes-2.0-flash", "minimal"), "low");
+  assert.equal(clamp("agnes-2.5-flash", "minimal"), "low");
+  assert.equal(clamp("agnes-2.5-flash", "off"), "none");
+});
+
 test("agnes registry advertises the live OpenAI-style /models endpoint", () => {
   const entry = providerRegistry.agnes;
   assert.equal(entry.modelsUrl, AGNES_MODELS_URL);
 });
 
 test("agnes is classified for live OpenAI-style /models discovery", async () => {
-  const { isNamedOpenAIStyleProvider } = await import(
-    "../../src/app/api/providers/[id]/models/discovery/providerSets.ts"
-  );
+  const { isNamedOpenAIStyleProvider } =
+    await import("../../src/app/api/providers/[id]/models/discovery/providerSets.ts");
   assert.equal(isNamedOpenAIStyleProvider("agnes"), true);
 });
 
@@ -125,9 +202,8 @@ test("agnes honors per-connection CN base URL override", () => {
 });
 
 test("agnes base-URL field is always-on so CN keys can point at api.agnes-ai.cn", async () => {
-  const helpers = await import(
-    "../../src/app/(dashboard)/dashboard/providers/[id]/providerPageHelpers.ts"
-  );
+  const helpers =
+    await import("../../src/app/(dashboard)/dashboard/providers/[id]/providerPageHelpers.ts");
   assert.equal(helpers.isBaseUrlConfigurableProvider("agnes"), true);
   assert.equal(helpers.getProviderBaseUrlDefault("agnes"), "https://apihub.agnes-ai.com/v1");
   assert.equal(helpers.getProviderBaseUrlPlaceholder("agnes"), AGNES_CN_BASE_URL);