diff --git a/docs/guides/USER_GUIDE.md b/docs/guides/USER_GUIDE.md
index 5a919867e7..dc0a51f747 100644
--- a/docs/guides/USER_GUIDE.md
+++ b/docs/guides/USER_GUIDE.md
@@ -144,6 +144,10 @@ Models:
 
 **Pro Tip:** Use Opus for complex tasks, Sonnet for speed. OmniRoute tracks quota per model!
 
+Claude and Claude Code-compatible routes preserve `max` thinking effort for Opus and Sonnet
+models. Haiku models do not accept the `max` effort tier, so OmniRoute downgrades that
+request to a high thinking budget before sending it upstream.
+
 #### OpenAI Codex (Plus/Pro)
 
 ```bash
diff --git a/open-sse/config/providerModels.ts b/open-sse/config/providerModels.ts
index c124984c91..99a42ea019 100644
--- a/open-sse/config/providerModels.ts
+++ b/open-sse/config/providerModels.ts
@@ -59,6 +59,20 @@ export function getModelsByProviderId(providerId: string): RegistryModel[] {
   return PROVIDER_MODELS[alias] || [];
 }
 
+const CLAUDE_MODEL_PATTERN = /(?:^|[\/._-])claude(?:[._-]|$)/;
+const CLAUDE_MAX_EFFORT_UNSUPPORTED_FAMILY_PATTERNS = [/(?:^|[\/._-])haiku(?:[._-]|$)/] as const;
+
+export function supportsClaudeMaxEffort(modelId: string | null | undefined): boolean {
+  if (typeof modelId !== "string" || modelId.length === 0) return false;
+  const normalized = modelId.toLowerCase();
+  const claudeMatch = normalized.match(CLAUDE_MODEL_PATTERN);
+  if (!claudeMatch) return false;
+  const claudeScopedId = normalized.slice(claudeMatch.index ?? 0);
+  return !CLAUDE_MAX_EFFORT_UNSUPPORTED_FAMILY_PATTERNS.some((pattern) =>
+    pattern.test(claudeScopedId)
+  );
+}
+
 export function supportsXHighEffort(aliasOrId: string, modelId: string): boolean {
   const alias = PROVIDER_ID_TO_ALIAS[aliasOrId] || aliasOrId;
   const providerModels = PROVIDER_MODELS[alias] || PROVIDER_MODELS[aliasOrId];
diff --git a/open-sse/executors/base.ts b/open-sse/executors/base.ts
index 388f85e0f7..f45c12fe68 100644
--- a/open-sse/executors/base.ts
+++ b/open-sse/executors/base.ts
@@ -1,6 +1,6 @@
 import { HTTP_STATUS, FETCH_TIMEOUT_MS } from "../config/constants.ts";
 import { applyFingerprint, isCliCompatEnabled } from "../config/cliFingerprints.ts";
-import { supportsXHighEffort } from "../config/providerModels.ts";
+import { supportsClaudeMaxEffort, supportsXHighEffort } from "../config/providerModels.ts";
 import {
   getRotatingApiKey,
   getValidApiKey,
@@ -205,8 +205,8 @@ function hasActiveClaudeThinking(body: Record<string, unknown>): boolean {
 /**
  * Sanitize reasoning_effort for providers that don't accept all values.
  *
- * The claude→openai translator emits reasoning_effort=xhigh when the client
- * sends output_config.effort=max on a Claude-shape request. Combined with
+ * The claude→openai translator may emit reasoning_effort=max/xhigh when the
+ * client sends output_config.effort=max on a Claude-shape request. Combined with
  * runtime alias remapping (e.g. claude-opus-4-6 → mimo/mimo-v2.5-pro), this
  * routes xhigh to OpenAI-shape providers that don't accept the value:
  *
@@ -217,11 +217,22 @@ function hasActiveClaudeThinking(body: Record<string, unknown>): boolean {
  * Each rejection burns a combo fallback attempt before reaching a working
  * provider. Apply provider-aware sanitation here (after transformRequest, so
  * reintroductions by per-provider transforms are also caught) before fetch.
- * Models that genuinely support xhigh pass through unchanged. Claude models
- * default to xhigh support unless they are marked as legacy unsupported entries.
+ * xhigh support is registry-gated: models that genuinely support xhigh pass
+ * through unchanged, and Claude models default to xhigh support unless marked
+ * as legacy unsupported entries. max support is Claude/CC-compatible only and
+ * intentionally separate: older Opus/Sonnet models may support max even when
+ * they do not support xhigh.
  */
 const MISTRAL_NO_REASONING_EFFORT_PATTERN = /devstral/i;
 const GITHUB_NO_REASONING_EFFORT_PATTERN = /(claude|haiku|oswe)/i;
+
+function supportsMaxEffortForProvider(provider: string, model: string): boolean {
+  return (
+    (provider === PROVIDER_CLAUDE || isClaudeCodeCompatible(provider)) &&
+    supportsClaudeMaxEffort(model)
+  );
+}
+
 export function sanitizeReasoningEffortForProvider(
   body: unknown,
   provider: string,
@@ -240,10 +251,14 @@ export function sanitizeReasoningEffortForProvider(
   const effortStr = typeof effort === "string" ? effort.toLowerCase() : "";
   const modelStr = model || "";
 
-  if (effortStr === "xhigh" && !supportsXHighEffort(provider, modelStr)) {
+  const shouldDowngradeXHigh = effortStr === "xhigh" && !supportsXHighEffort(provider, modelStr);
+  const shouldDowngradeMax =
+    effortStr === "max" && !supportsMaxEffortForProvider(provider, modelStr);
+
+  if (shouldDowngradeXHigh || shouldDowngradeMax) {
     log?.info?.(
       "REASONING_SANITIZE",
-      `${provider}/${modelStr}: downgraded reasoning_effort xhigh → high`
+      `${provider}/${modelStr}: downgraded reasoning_effort ${effortStr} → high`
     );
     const next: Record<string, unknown> = { ...b };
     if (hasTopLevelReasoningEffort) {
@@ -760,7 +775,7 @@ export class BaseExecutor {
           }
 
           // Per-request behavior overrides via custom client headers.
-          //   x-omniroute-effort:   low | medium | high | xhigh | off
+          //   x-omniroute-effort:   low | medium | high | xhigh | max | off
           //   x-omniroute-thinking: adaptive | off
           // A header value applies only when the corresponding body field is
           // not already set; "off" force-strips the field.
@@ -782,7 +797,7 @@ export class BaseExecutor {
               delete (tb.output_config as Record<string, unknown>).effort;
             }
             appliedEffort = "off";
-          } else if (headerEffort && ["low", "medium", "high", "xhigh"].includes(headerEffort)) {
+          } else if (headerEffort && ["low", "medium", "high", "xhigh", "max"].includes(headerEffort)) {
             const oc =
               tb.output_config && typeof tb.output_config === "object"
                 ? (tb.output_config as Record<string, unknown>)
diff --git a/open-sse/services/claudeCodeCompatible.ts b/open-sse/services/claudeCodeCompatible.ts
index 4972f21835..c7c6800687 100644
--- a/open-sse/services/claudeCodeCompatible.ts
+++ b/open-sse/services/claudeCodeCompatible.ts
@@ -2,7 +2,7 @@ import { createHash, randomUUID } from "node:crypto";
 
 import { getStainlessTimeoutSeconds } from "@/shared/utils/runtimeTimeouts";
 import { ANTHROPIC_VERSION_HEADER } from "../config/anthropicHeaders.ts";
-import { supportsXHighEffort } from "../config/providerModels.ts";
+import { supportsClaudeMaxEffort, supportsXHighEffort } from "../config/providerModels.ts";
 import { prepareClaudeRequest } from "../translator/helpers/claudeHelper.ts";
 import { signRequestBody } from "./claudeCodeCCH.ts";
 import { remapToolNamesInRequest } from "./claudeCodeToolRemapper.ts";
@@ -451,7 +451,7 @@ export function resolveClaudeCodeCompatibleEffort(
   sourceBody?: Record<string, unknown> | null,
   normalizedBody?: Record<string, unknown> | null,
   model?: string | null
-): "low" | "medium" | "high" | "xhigh" {
+): "low" | "medium" | "high" | "xhigh" | "max" {
   const raw =
     readNestedString(sourceBody, ["output_config", "effort"]) ||
     readNestedString(sourceBody, ["reasoning", "effort"]) ||
@@ -474,7 +474,7 @@ export function resolveClaudeCodeCompatibleEffort(
     return supportsClaudeXHighEffort(model) ? "xhigh" : "high";
   }
   if (normalizedEffort === "max") {
-    return supportsClaudeXHighEffort(model) ? "xhigh" : "high";
+    return supportsClaudeMaxEffort(model) ? "max" : "high";
   }
   return supportsClaudeXHighEffort(model) ? "xhigh" : "high";
 }
@@ -1102,7 +1102,7 @@ function resolveClaudeCodeCompatibleOutputConfig({
   sourceBody?: Record<string, unknown> | null;
   normalizedBody?: Record<string, unknown> | null;
   model?: string | null;
-  effort: "low" | "medium" | "high" | "xhigh";
+  effort: "low" | "medium" | "high" | "xhigh" | "max";
 }) {
   const outputConfig =
     readRecord(cloneValue(claudeBody?.output_config)) ||
diff --git a/open-sse/translator/request/openai-to-claude.ts b/open-sse/translator/request/openai-to-claude.ts
index bbdc3a894c..2671d7a5e8 100644
--- a/open-sse/translator/request/openai-to-claude.ts
+++ b/open-sse/translator/request/openai-to-claude.ts
@@ -1,7 +1,7 @@
 import { register } from "../registry.ts";
 import { FORMATS } from "../formats.ts";
 // CLAUDE_SYSTEM_PROMPT import removed — no longer injected unconditionally (#1966/#2130)
-import { supportsXHighEffort } from "../../config/providerModels.ts";
+import { supportsClaudeMaxEffort, supportsXHighEffort } from "../../config/providerModels.ts";
 import { adjustMaxTokens } from "../helpers/maxTokensHelper.ts";
 import { sanitizeToolId } from "../helpers/schemaCoercion.ts";
 import { DEFAULT_THINKING_CLAUDE_SIGNATURE } from "../../config/defaultThinkingSignature.ts";
@@ -453,16 +453,18 @@ export function openaiToClaudeRequest(model, body, stream) {
     // Clients like OpenCode send reasoning_effort via @ai-sdk/openai-compatible
     const requestedEffort = String(body.reasoning_effort).toLowerCase();
     const normalizedEffort =
-      requestedEffort === "xhigh" && !supportsXHighEffort("claude", model)
+      requestedEffort === "max" && !supportsClaudeMaxEffort(model)
         ? "high"
-        : requestedEffort;
-    if (normalizedEffort === "xhigh") {
+        : requestedEffort === "xhigh" && !supportsXHighEffort("claude", model)
+          ? "high"
+          : requestedEffort;
+    if (normalizedEffort === "max" || normalizedEffort === "xhigh") {
       result.thinking = {
         type: "adaptive",
       };
       result.output_config = {
         ...(result.output_config || {}),
-        effort: "xhigh",
+        effort: normalizedEffort,
       };
     } else {
       const effortBudgetMap: Record<string, number> = {
diff --git a/tests/unit/base-executor-sanitize-effort.test.ts b/tests/unit/base-executor-sanitize-effort.test.ts
index abb0930595..910e1c8fd6 100644
--- a/tests/unit/base-executor-sanitize-effort.test.ts
+++ b/tests/unit/base-executor-sanitize-effort.test.ts
@@ -28,6 +28,65 @@ test("sanitizeReasoningEffortForProvider: xiaomi-mimo downgrades xhigh → high"
   );
 });
 
+test("sanitizeReasoningEffortForProvider: xiaomi-mimo downgrades max → high", () => {
+  const log = makeLog();
+  const body = {
+    model: "mimo-v2.5-pro",
+    reasoning_effort: "max",
+    messages: [{ role: "user", content: "hi" }],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "xiaomi-mimo", "mimo-v2.5-pro", log);
+  assert.equal((result as any).reasoning_effort, "high");
+  assert.ok(
+    log.messages.some(([tag, m]) => tag === "REASONING_SANITIZE" && /max → high/.test(m)),
+    "logs the downgrade"
+  );
+});
+
+test("sanitizeReasoningEffortForProvider: claude preserves max for Opus/Sonnet and downgrades Haiku", () => {
+  const sonnetBody = {
+    model: "claude-sonnet-4-6",
+    reasoning_effort: "max",
+    messages: [{ role: "user", content: "hi" }],
+  };
+  const sonnetResult = sanitizeReasoningEffortForProvider(
+    sonnetBody,
+    "claude",
+    "claude-sonnet-4-6",
+    null
+  );
+  assert.equal(sonnetResult, sonnetBody);
+  assert.equal((sonnetResult as any).reasoning_effort, "max");
+
+  const opusBody = {
+    model: "claude-opus-4-6",
+    reasoning: { effort: "max", summary: "auto" },
+    input: [],
+  };
+  const opusResult = sanitizeReasoningEffortForProvider(
+    opusBody,
+    "anthropic-compatible-cc-test",
+    "claude-opus-4-6",
+    null
+  );
+  assert.equal(opusResult, opusBody);
+  assert.equal((opusResult as any).reasoning.effort, "max");
+
+  const haikuBody = {
+    model: "claude-haiku-4-5-20251001",
+    reasoning_effort: "max",
+    messages: [{ role: "user", content: "hi" }],
+  };
+  const haikuResult = sanitizeReasoningEffortForProvider(
+    haikuBody,
+    "claude",
+    "claude-haiku-4-5-20251001",
+    null
+  );
+  assert.notEqual(haikuResult, haikuBody);
+  assert.equal((haikuResult as any).reasoning_effort, "high");
+});
+
 test("sanitizeReasoningEffortForProvider: xiaomi-mimo downgrades xhigh in nested reasoning.effort", () => {
   const body = {
     model: "mimo-v2.5-pro",
diff --git a/tests/unit/claude-code-compatible-request.test.ts b/tests/unit/claude-code-compatible-request.test.ts
index 13ece0f7b3..895693b65f 100644
--- a/tests/unit/claude-code-compatible-request.test.ts
+++ b/tests/unit/claude-code-compatible-request.test.ts
@@ -53,6 +53,10 @@ test("Claude Code compatible effort and max token helpers cover priority fallbac
     resolveClaudeCodeCompatibleEffort({ output_config: { effort: "xhigh" } }, null, xhighModel.id),
     "xhigh"
   );
+  assert.equal(
+    resolveClaudeCodeCompatibleEffort({ output_config: { effort: "max" } }, null, xhighModel.id),
+    "max"
+  );
   assert.equal(
     resolveClaudeCodeCompatibleEffort(
       { output_config: { effort: "xhigh" } },
@@ -61,6 +65,18 @@ test("Claude Code compatible effort and max token helpers cover priority fallbac
     ),
     "high"
   );
+  assert.equal(
+    resolveClaudeCodeCompatibleEffort({ output_config: { effort: "max" } }, null, standardModel.id),
+    "max"
+  );
+  assert.equal(
+    resolveClaudeCodeCompatibleEffort(
+      { output_config: { effort: "max" } },
+      null,
+      "claude-haiku-4-5-20251001"
+    ),
+    "high"
+  );
   assert.equal(
     resolveClaudeCodeCompatibleEffort({ output_config: { effort: "unexpected" } }),
     "high"
diff --git a/tests/unit/provider-models-config.test.ts b/tests/unit/provider-models-config.test.ts
index d992860f3e..0ebd754469 100644
--- a/tests/unit/provider-models-config.test.ts
+++ b/tests/unit/provider-models-config.test.ts
@@ -10,6 +10,7 @@ import {
   getModelsByProviderId,
   getProviderModels,
   isValidModel,
+  supportsClaudeMaxEffort,
   supportsXHighEffort,
 } from "../../open-sse/config/providerModels.ts";
 
@@ -92,6 +93,19 @@ test("Kiro registry exposes the current CLI model lineup with context windows",
   assert.equal(byId.has("claude-haiku-4-5"), false);
 });
 
+test("Claude max effort support excludes Haiku family and non-Claude IDs", () => {
+  assert.equal(supportsClaudeMaxEffort("claude-opus-4-7"), true);
+  assert.equal(supportsClaudeMaxEffort("claude-opus-4-6"), true);
+  assert.equal(supportsClaudeMaxEffort("claude-sonnet-4-6"), true);
+  assert.equal(supportsClaudeMaxEffort("claude-sonnet-4-5-20250929"), true);
+  assert.equal(supportsClaudeMaxEffort("claude-haiku-4-5-20251001"), false);
+  assert.equal(supportsClaudeMaxEffort("claude-3-5-haiku-20241022"), false);
+  assert.equal(supportsClaudeMaxEffort("anthropic/claude-haiku-4.5"), false);
+  assert.equal(supportsClaudeMaxEffort("vendor/haiku-compatible-claude-sonnet-4-6"), true);
+  assert.equal(supportsClaudeMaxEffort("gpt-5"), false);
+  assert.equal(supportsClaudeMaxEffort("claude-future-5-0"), true);
+});
+
 test("Claude xhigh effort support defaults on for new models and opts out legacy models", () => {
   const claudeModels = new Set(getModelsByProviderId("claude").map((model) => model.id));
 
diff --git a/tests/unit/translator-openai-to-claude.test.ts b/tests/unit/translator-openai-to-claude.test.ts
index 36ab95f3f2..1bf5ca456a 100644
--- a/tests/unit/translator-openai-to-claude.test.ts
+++ b/tests/unit/translator-openai-to-claude.test.ts
@@ -307,6 +307,32 @@ test("OpenAI -> Claude preserves xhigh only for Claude models that expose it", (
   assert.equal(downgraded.max_tokens, 128000);
 });
 
+test("OpenAI -> Claude preserves max effort except for Haiku models", () => {
+  const preserved = openaiToClaudeRequest(
+    "claude-sonnet-4-6",
+    {
+      messages: [{ role: "user", content: "Think at max" }],
+      reasoning_effort: "max",
+    },
+    false
+  );
+  const haiku = openaiToClaudeRequest(
+    "claude-haiku-4-5-20251001",
+    {
+      messages: [{ role: "user", content: "Think at max" }],
+      max_tokens: 10,
+      reasoning_effort: "max",
+    },
+    false
+  );
+
+  assert.deepEqual(preserved.thinking, { type: "adaptive" });
+  assert.deepEqual(preserved.output_config, { effort: "max" });
+  assert.equal(haiku.output_config, undefined);
+  assert.deepEqual(haiku.thinking, { type: "enabled", budget_tokens: 62976 });
+  assert.equal(haiku.max_tokens, 64000);
+});
+
 test("OpenAI -> Claude fits thinking budget within Opus 4.7 output cap (regression)", () => {
   // Real-world OpenCode scenario: caller asks for max_tokens=32000 with high effort.
   // High effort maps to budget=131072. The previous naive