diff --git a/.changeset/anthropic-max-tokens-fallback.md b/.changeset/anthropic-max-tokens-fallback.md
new file mode 100644
index 0000000000..71c9e64db6
--- /dev/null
+++ b/.changeset/anthropic-max-tokens-fallback.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix 400 rejections from Anthropic-compatible providers that enforce a lower max_tokens output limit than the default 128000 fallback. Custom endpoints now use a conservative 32768 default, and a rejected request is automatically retried with the provider's declared limit.
diff --git a/packages/agent-core-v2/src/agent/llmRequester/llmRequesterService.ts b/packages/agent-core-v2/src/agent/llmRequester/llmRequesterService.ts
index e8589fdd74..07c2d0e8c0 100644
--- a/packages/agent-core-v2/src/agent/llmRequester/llmRequesterService.ts
+++ b/packages/agent-core-v2/src/agent/llmRequester/llmRequesterService.ts
@@ -8,8 +8,10 @@
  * params, then drives a bounded request chain through the `ModelRequester`
  * resolved from `IModelCatalog`: one primary `requester.request(input, signal,
  * params)` attempt plus projection rebuilds for request structure or media
- * compatibility; general retry policy remains in the loop's `stepRetry`
- * plugin. Before each request the projected messages pass through `media`'s
+ * compatibility and a one-shot completion-budget clamp when the provider
+ * rejects `max_tokens` (the server-declared ceiling is remembered per model,
+ * so each model pays for at most one rejected-and-resent request); general
+ * retry policy remains in the loop's `stepRetry` plugin. Before each request the projected messages pass through `media`'s
  * video resolver, which rewrites every `kimi-file://` prompt-video reference
  * to a provider-acceptable part (uploaded `ms://`, inline base64, or a
  * `<video path>` tag) so the internal reference never reaches the wire. When a
@@ -53,6 +55,7 @@ import {
   isImageFormatError,
   isRecoverableRequestStructureError,
   isRetryableGenerateError,
+  parseMaxTokensLimit,
 } from '#/kosong/contract/errors';
 import { type Message } from '#/kosong/contract/message';
 import { type ThinkingEffort } from '#/kosong/contract/provider';
@@ -147,6 +150,8 @@ export class AgentLLMRequesterService implements IAgentLLMRequesterService {
   private readonly mediaDegradedTurns = new Set<number>();
   private readonly mediaStrippedTurns = new Map<number, MediaStripSnapshot>();
   private readonly emittedThinkingEffortWarnings = new Set<string>();
+  /** Server-declared `max_tokens` ceilings learned from a rejected request, keyed by wire model name. */
+  private readonly maxTokensClamps = new Map<string, number>();
 
   constructor(
     @IAgentContextMemoryService private readonly context: IAgentContextMemoryService,
@@ -284,6 +289,7 @@ export class AgentLLMRequesterService implements IAgentLLMRequesterService {
   ): Promise<AgentLLMRequestFinish> {
     const shaped = this.toolSelect.shapeHistory(request.messages);
     let mediaStripSnapshot = this.mediaStripSnapshotForTurn(request.source);
+    let params = this.applyMaxTokensClamp(request.model.name, request.params);
     const requestInput = (projection: RequestProjection) => {
       return {
         systemPrompt: request.systemPrompt,
@@ -325,7 +331,7 @@ export class AgentLLMRequesterService implements IAgentLLMRequesterService {
         modelName: request.model.name,
         modelAlias: request.modelAlias,
         thinkingEffort: request.thinkingEffort,
-        maxTokens: effectiveMaxCompletionTokens(request.params),
+        maxTokens: effectiveMaxCompletionTokens(params),
         systemPrompt: input.systemPrompt,
         tools: input.tools,
         messages: input.messages,
@@ -350,7 +356,7 @@ export class AgentLLMRequesterService implements IAgentLLMRequesterService {
       };
 
       for await (const event of request.requester.request(input, signal, {
-        ...request.params,
+        ...params,
         onTraceId: setTraceId,
       })) {
         switch (event.type) {
@@ -457,6 +463,26 @@ export class AgentLLMRequesterService implements IAgentLLMRequesterService {
           projection = 'strict';
           continue;
         }
+        if (raw instanceof APIStatusError && raw.statusCode === 400) {
+          const maxTokensLimit = parseMaxTokensLimit(raw.message);
+          if (
+            maxTokensLimit !== null &&
+            maxTokensLimit < (params.maxCompletionTokens ?? Number.POSITIVE_INFINITY)
+          ) {
+            signal?.throwIfAborted();
+            this.log.warn(
+              'provider rejected the completion-token budget; resending with clamped max tokens',
+              {
+                model: request.model.name,
+                maxTokens: maxTokensLimit,
+                ...request.logFields,
+              },
+            );
+            this.maxTokensClamps.set(request.model.name, maxTokensLimit);
+            params = { ...params, maxCompletionTokens: maxTokensLimit };
+            continue;
+          }
+        }
         throw error;
       }
     }
@@ -526,6 +552,15 @@ export class AgentLLMRequesterService implements IAgentLLMRequesterService {
     set.add(source.turnId);
   }
 
+  private applyMaxTokensClamp(modelName: string, params: ModelRequestParams): ModelRequestParams {
+    const clamp = this.maxTokensClamps.get(modelName);
+    if (clamp === undefined) return params;
+    return {
+      ...params,
+      maxCompletionTokens: Math.min(clamp, params.maxCompletionTokens ?? clamp),
+    };
+  }
+
   private resolveRequest(overrides: AgentLLMRequestOverrides): ResolvedLLMRequest {
     const turnConfig = this.resolveTurnConfig(overrides.source);
     const resolved = turnConfig?.resolved ?? this.profile.resolveModelContext();
diff --git a/packages/agent-core-v2/src/kosong/contract/errors.ts b/packages/agent-core-v2/src/kosong/contract/errors.ts
index 43b9ee5dd7..2651d46803 100644
--- a/packages/agent-core-v2/src/kosong/contract/errors.ts
+++ b/packages/agent-core-v2/src/kosong/contract/errors.ts
@@ -255,6 +255,34 @@ const CONTEXT_OVERFLOW_MESSAGE_PATTERNS = [
   /request.*exceed(?:ed|s|ing)?.*model token limit/,
 ] as const;
 
+const MAX_TOKENS_LIMIT_MESSAGE_PATTERNS: readonly RegExp[] = [
+  /max_tokens[\s\S]{0,200}expected a value\s*<=\s*(\d+)/i,
+  /max_tokens[\s\S]{0,200}(?:must be at most|cannot exceed|maximum value[^\d]*)\s*(\d+)/i,
+  /max_tokens[\s\S]{0,200}(?:less than or equal to|<=)\s*(\d+)/i,
+  /max_tokens[\s\S]{0,200}max output tokens is\s*(\d+)/i,
+];
+
+/**
+ * Parse the provider's actual max_tokens ceiling from a 400 error message.
+ * Returns the parsed limit, or null if no max_tokens limit pattern matched.
+ * Patterns are matched against the full error message text (which includes
+ * the provider's JSON body) and each anchors on `max_tokens` to avoid false
+ * positives from context-overflow messages. Recognized wordings: volcano/ark
+ * "expected a value <= N"; "must be at most N" / "cannot exceed N" /
+ * "maximum value … N"; "must be less than or equal to N" and the bare "<= N"
+ * form; Anthropic-native "max output tokens is N".
+ */
+export function parseMaxTokensLimit(message: string): number | null {
+  for (const pattern of MAX_TOKENS_LIMIT_MESSAGE_PATTERNS) {
+    const match = pattern.exec(message);
+    if (match?.[1]) {
+      const limit = parseInt(match[1], 10);
+      if (Number.isFinite(limit) && limit > 0) return limit;
+    }
+  }
+  return null;
+}
+
 const PROVIDER_RATE_LIMIT_MESSAGE_PATTERNS = [
   /(?:apistatuserror.*429|429.*apistatuserror)/,
   /429.*too many requests/,
diff --git a/packages/agent-core-v2/src/kosong/provider/bases/anthropic/anthropic.ts b/packages/agent-core-v2/src/kosong/provider/bases/anthropic/anthropic.ts
index fc276bb066..29e4810fcb 100644
--- a/packages/agent-core-v2/src/kosong/provider/bases/anthropic/anthropic.ts
+++ b/packages/agent-core-v2/src/kosong/provider/bases/anthropic/anthropic.ts
@@ -207,6 +207,8 @@ const CEILING_BY_FAMILY_VERSION: Readonly<Record<string, number>> = {
 };
 
 const FALLBACK_MAX_TOKENS = 128000;
+/** Conservative fallback for custom endpoints whose actual output ceiling is unknown. */
+const CUSTOM_ENDPOINT_FALLBACK_MAX_TOKENS = 32768;
 
 function lookupClaudeCeiling(version: AnthropicModelVersion): number | undefined {
   const { family, major, minor } = version;
@@ -219,11 +221,18 @@ function lookupClaudeCeiling(version: AnthropicModelVersion): number | undefined
   return CEILING_BY_FAMILY_VERSION[`${family}-${major}`];
 }
 
-export function resolveDefaultMaxTokens(model: string, override?: number): number {
+export function resolveDefaultMaxTokens(model: string, override?: number, baseUrl?: string): number {
   const parsed = parseAnthropicModelVersion(model, true);
   const ceiling = parsed === null ? undefined : lookupClaudeCeiling(parsed);
   if (ceiling === undefined) {
-    return override ?? FALLBACK_MAX_TOKENS;
+    if (override !== undefined) return override;
+    // Custom endpoint + unrecognized model: the 128k Claude-4 fallback is too
+    // aggressive for most Anthropic-compatible providers (32768 is the most
+    // common ceiling). Use the conservative value; the user can still override
+    // via defaultMaxTokens (max_output_size in config).
+    const isCustomEndpoint =
+      baseUrl !== undefined && baseUrl !== '' && !baseUrl.includes('api.anthropic.com');
+    return isCustomEndpoint ? CUSTOM_ENDPOINT_FALLBACK_MAX_TOKENS : FALLBACK_MAX_TOKENS;
   }
   return override === undefined ? ceiling : Math.min(override, ceiling);
 }
@@ -821,7 +830,9 @@ export class AnthropicChatProvider implements ChatProvider {
     this._client = this._apiKey === undefined ? undefined : this._buildClient(this._apiKey);
     this._explicitMaxTokens = options.defaultMaxTokens !== undefined;
     this._generationKwargs = {
-      max_tokens: options.defaultMaxTokens ?? resolveDefaultMaxTokens(options.model),
+      max_tokens:
+        options.defaultMaxTokens ??
+        resolveDefaultMaxTokens(options.model, undefined, options.baseUrl),
       betaFeatures: options.betaFeatures ?? [INTERLEAVED_THINKING_BETA],
     };
   }
@@ -927,7 +938,7 @@ export class AnthropicChatProvider implements ChatProvider {
         cap = Math.min(cap, options.maxContextTokens - options.usedContextTokens);
       }
       cap = Math.max(1, cap);
-      const requestedCap = resolveDefaultMaxTokens(this._model, cap);
+      const requestedCap = resolveDefaultMaxTokens(this._model, cap, this._baseUrl);
       const existingCap = kwargs.max_tokens;
       kwargs = {
         ...kwargs,
diff --git a/packages/agent-core-v2/test/agent/llmRequester/llmRequesterService.test.ts b/packages/agent-core-v2/test/agent/llmRequester/llmRequesterService.test.ts
index a91ddfb3df..3fb0e5ba17 100644
--- a/packages/agent-core-v2/test/agent/llmRequester/llmRequesterService.test.ts
+++ b/packages/agent-core-v2/test/agent/llmRequester/llmRequesterService.test.ts
@@ -51,6 +51,7 @@ import { IModelCatalog, type Model } from '#/kosong/model/catalog';
 import {
   type ModelRequestEvent,
   type ModelRequestInput,
+  type ModelRequestParams,
   type ModelRequester,
 } from '#/kosong/model/modelRequester';
 import { ITelemetryService } from '#/app/telemetry/telemetry';
@@ -616,6 +617,97 @@ describe('AgentLLMRequesterService media-degraded resend', () => {
   });
 });
 
+describe('AgentLLMRequesterService max-tokens clamp resend', () => {
+  // The harness budget is the stub model's 1000-token context window, so the
+  // server-declared ceiling must sit below it for the clamp to lower anything.
+  const MAX_TOKENS_400 = new APIStatusError(400, 'max_tokens: 128000, expected a value <= 512');
+
+  function createMaxTokensRequester(
+    calls: { value: number },
+    capturedParams: Array<ModelRequestParams | undefined>,
+    errors: readonly (Error | undefined)[],
+  ): ModelRequester {
+    const model: Model = {
+      id: 'm',
+      name: 'wire-model',
+      aliases: [],
+      protocol: 'anthropic',
+      baseUrl: 'https://example.test',
+      headers: {},
+      capabilities,
+      maxContextSize: 1000,
+      alwaysThinking: false,
+      providerName: 'p',
+      authProvider: { getAuth: async () => undefined },
+    };
+    return {
+      model,
+      request: async function* (_input, _signal, params) {
+        const index = calls.value;
+        calls.value += 1;
+        capturedParams.push(params);
+        const error = errors[index];
+        if (error !== undefined) throw error;
+        yield {
+          type: 'finish',
+          message: { role: 'assistant', content: [{ type: 'text', text: 'ok' }], toolCalls: [] },
+          providerFinishReason: 'completed',
+          rawFinishReason: 'stop',
+          id: 'resp-1',
+        };
+      },
+    };
+  }
+
+  it('resends once with the server-declared max_tokens ceiling', async () => {
+    const calls = { value: 0 };
+    const capturedParams: Array<ModelRequestParams | undefined> = [];
+    const requester = createMaxTokensRequester(calls, capturedParams, [
+      MAX_TOKENS_400,
+      undefined,
+    ]);
+    const { service } = createService(requester, undefined);
+
+    const result = await service.request();
+
+    expect(result.message.content).toEqual([{ type: 'text', text: 'ok' }]);
+    expect(calls.value).toBe(2);
+    expect(capturedParams[0]?.maxCompletionTokens).toBeGreaterThan(512);
+    expect(capturedParams[1]?.maxCompletionTokens).toBe(512);
+  });
+
+  it('stops after one clamp resend when the clamped request is rejected again', async () => {
+    const calls = { value: 0 };
+    const capturedParams: Array<ModelRequestParams | undefined> = [];
+    const requester = createMaxTokensRequester(calls, capturedParams, [
+      MAX_TOKENS_400,
+      MAX_TOKENS_400,
+    ]);
+    const { service } = createService(requester, undefined);
+
+    await expect(service.request()).rejects.toMatchObject({ statusCode: 400 });
+    expect(calls.value).toBe(2);
+  });
+
+  it('applies the remembered clamp to later requests for the same model', async () => {
+    const calls = { value: 0 };
+    const capturedParams: Array<ModelRequestParams | undefined> = [];
+    const requester = createMaxTokensRequester(calls, capturedParams, [
+      MAX_TOKENS_400,
+      undefined,
+      undefined,
+    ]);
+    const { service } = createService(requester, undefined);
+
+    await service.request({ source: { type: 'turn', turnId: 1, step: 1 } });
+    expect(calls.value).toBe(2);
+
+    await service.request({ source: { type: 'turn', turnId: 2, step: 1 } });
+    expect(calls.value).toBe(3);
+    expect(capturedParams[2]?.maxCompletionTokens).toBe(512);
+  });
+});
+
 describe('AgentLLMRequesterService fault injection (experimental)', () => {
   it('raises an armed request-too-large fault before the provider and recovers via the degraded resend', async () => {
     const calls = { value: 0 };
diff --git a/packages/agent-core-v2/test/kosong/contract/errors.test.ts b/packages/agent-core-v2/test/kosong/contract/errors.test.ts
index b4ba79cef8..20e3d49128 100644
--- a/packages/agent-core-v2/test/kosong/contract/errors.test.ts
+++ b/packages/agent-core-v2/test/kosong/contract/errors.test.ts
@@ -23,6 +23,7 @@ import {
   isAbortError,
   isRetryableGenerateError,
   normalizeAPIStatusError,
+  parseMaxTokensLimit,
   throwIfAbortError,
 } from '#/kosong/contract/errors';
 
@@ -183,3 +184,30 @@ describe('classifyApiError', () => {
     expect(classifyApiError('boom').kind).toBe('other');
   });
 });
+
+describe('parseMaxTokensLimit', () => {
+  it('parses the "expected a value <= N" form', () => {
+    expect(parseMaxTokensLimit('max_tokens: 128000, expected a value <= 8192')).toBe(8192);
+  });
+
+  it('parses the "must be at most", "cannot exceed", and "maximum value" forms', () => {
+    expect(parseMaxTokensLimit('max_tokens: value must be at most 32768')).toBe(32768);
+    expect(parseMaxTokensLimit('max_tokens cannot exceed 16384 for this model')).toBe(16384);
+    expect(parseMaxTokensLimit('max_tokens: maximum value is 64000')).toBe(64000);
+  });
+
+  it('parses the "max output tokens is N" form case-insensitively', () => {
+    expect(parseMaxTokensLimit('Max_Tokens: 99999 but max output tokens is 12000')).toBe(12000);
+  });
+
+  it('parses the "less than or equal to" and bare "<=" forms', () => {
+    expect(parseMaxTokensLimit('max_tokens must be less than or equal to 4096')).toBe(4096);
+    expect(parseMaxTokensLimit('max_tokens <= 2048')).toBe(2048);
+  });
+
+  it('returns null when no recognizable positive limit is present', () => {
+    expect(parseMaxTokensLimit('max_tokens must be positive')).toBeNull();
+    expect(parseMaxTokensLimit('some unrelated validation problem')).toBeNull();
+    expect(parseMaxTokensLimit('max_tokens: expected a value <= 0')).toBeNull();
+  });
+});
diff --git a/packages/agent-core-v2/test/kosong/provider/composition.test.ts b/packages/agent-core-v2/test/kosong/provider/composition.test.ts
index e6293bd978..a312f074c5 100644
--- a/packages/agent-core-v2/test/kosong/provider/composition.test.ts
+++ b/packages/agent-core-v2/test/kosong/provider/composition.test.ts
@@ -949,6 +949,28 @@ describe('Anthropic max-tokens profile', () => {
     expect(resolveDefaultMaxTokens('totally-unknown-model')).toBe(128000);
   });
 
+  it('defaults unknown models on custom endpoints to a conservative ceiling', () => {
+    expect(
+      resolveDefaultMaxTokens('totally-unknown-model', undefined, 'https://proxy.example.test/v1'),
+    ).toBe(32768);
+  });
+
+  it('keeps the 128000 fallback for unknown models on the official endpoint or no endpoint', () => {
+    expect(
+      resolveDefaultMaxTokens('totally-unknown-model', undefined, 'https://api.anthropic.com'),
+    ).toBe(128000);
+    expect(resolveDefaultMaxTokens('totally-unknown-model', undefined, '')).toBe(128000);
+  });
+
+  it('does not let a custom endpoint displace an explicit override or a known ceiling', () => {
+    expect(resolveDefaultMaxTokens('unknown-model', 12345, 'https://proxy.example.test/v1')).toBe(
+      12345,
+    );
+    expect(
+      resolveDefaultMaxTokens('claude-opus-4-7', undefined, 'https://proxy.example.test/v1'),
+    ).toBe(128000);
+  });
+
   it('sends the profile default as max_tokens when no explicit defaultMaxTokens is set', async () => {
     const provider = new AnthropicChatProvider({
       model: 'claude-opus-4-7',
@@ -999,6 +1021,19 @@ describe('Anthropic max-tokens profile', () => {
 
     expect(params['max_tokens']).toBe(999999);
   });
+
+  it('seeds the conservative fallback for unknown models on custom endpoints', async () => {
+    const provider = new AnthropicChatProvider({
+      model: 'totally-unknown-model',
+      apiKey: 'sk-probe',
+      baseUrl: 'https://proxy.example.test/v1',
+      stream: false,
+    });
+
+    const { params } = await captureAnthropicBody(provider);
+
+    expect(params['max_tokens']).toBe(32768);
+  });
 });
 
 describe('OpenAI reasoning_effort path (issue #1616)', () => {
diff --git a/packages/kosong/src/errors.ts b/packages/kosong/src/errors.ts
index 0bdb54b96c..fa5b6ee7e0 100644
--- a/packages/kosong/src/errors.ts
+++ b/packages/kosong/src/errors.ts
@@ -323,6 +323,38 @@ const CONTEXT_OVERFLOW_MESSAGE_PATTERNS = [
   /request.*exceed(?:ed|s|ing)?.*model token limit/,
 ] as const;
 
+/**
+ * Messages from providers that reject `max_tokens` above their actual output
+ * ceiling. Matched against the full error message text (which includes the
+ * provider's JSON body). Each pattern should anchor on `max_tokens` to avoid
+ * false positives from context-overflow messages.
+ */
+const MAX_TOKENS_LIMIT_MESSAGE_PATTERNS: readonly RegExp[] = [
+  // volcano / ark: "expected a value <= 32768"
+  /max_tokens[\s\S]{0,200}expected a value\s*<=\s*(\d+)/i,
+  // generic: "max_tokens must be at most N" / "max_tokens cannot exceed N"
+  /max_tokens[\s\S]{0,200}(?:must be at most|cannot exceed|maximum value[^\d]*)\s*(\d+)/i,
+  // "must be less than or equal to N" / bare "<= N" form
+  /max_tokens[\s\S]{0,200}(?:less than or equal to|<=)\s*(\d+)/i,
+  // anthropic native: "max_tokens: <model> max output tokens is N"
+  /max_tokens[\s\S]{0,200}max output tokens is\s*(\d+)/i,
+];
+
+/**
+ * Parse the provider's actual max_tokens ceiling from a 400 error message.
+ * Returns the parsed limit, or null if no max_tokens limit pattern matched.
+ */
+export function parseMaxTokensLimit(message: string): number | null {
+  for (const pattern of MAX_TOKENS_LIMIT_MESSAGE_PATTERNS) {
+    const match = pattern.exec(message);
+    if (match?.[1]) {
+      const limit = parseInt(match[1], 10);
+      if (Number.isFinite(limit) && limit > 0) return limit;
+    }
+  }
+  return null;
+}
+
 const PROVIDER_RATE_LIMIT_MESSAGE_PATTERNS = [
   /(?:apistatuserror.*429|429.*apistatuserror)/,
   /429.*too many requests/,
diff --git a/packages/kosong/src/providers/anthropic.ts b/packages/kosong/src/providers/anthropic.ts
index 6b34aa5343..9bf46006c2 100644
--- a/packages/kosong/src/providers/anthropic.ts
+++ b/packages/kosong/src/providers/anthropic.ts
@@ -222,6 +222,8 @@ const CEILING_BY_FAMILY_VERSION: Readonly<Record<string, number>> = {
 };
 
 const FALLBACK_MAX_TOKENS = 128000;
+/** Conservative fallback for custom endpoints whose actual output ceiling is unknown. */
+const CUSTOM_ENDPOINT_FALLBACK_MAX_TOKENS = 32768;
 
 function lookupClaudeCeiling(version: AnthropicModelVersion): number | undefined {
   const { family, major, minor } = version;
@@ -252,13 +254,22 @@ function lookupClaudeCeiling(version: AnthropicModelVersion): number | undefined
  *      the override is clamped to the documented Messages-API ceiling
  *      so we never send a value the server would reject.
  *   3. With no override and no recognized version, fall back to
- *      {@link FALLBACK_MAX_TOKENS}.
+ *      {@link FALLBACK_MAX_TOKENS} — or the conservative
+ *      {@link CUSTOM_ENDPOINT_FALLBACK_MAX_TOKENS} when `baseUrl` points at a
+ *      custom (non-Anthropic) endpoint, whose actual output ceiling is unknown
+ *      and is most commonly 32768.
  */
-export function resolveDefaultMaxTokens(model: string, override?: number): number {
+export function resolveDefaultMaxTokens(model: string, override?: number, baseUrl?: string): number {
   const parsed = parseAnthropicModelVersion(model, true);
   const ceiling = parsed === null ? undefined : lookupClaudeCeiling(parsed);
   if (ceiling === undefined) {
-    return override ?? FALLBACK_MAX_TOKENS;
+    if (override !== undefined) return override;
+    // Custom endpoint + unrecognized model: the 128k Claude-4 fallback is too
+    // aggressive for most Anthropic-compatible providers (32768 is the most
+    // common ceiling). Use the conservative value; the user can still override
+    // via defaultMaxTokens (max_output_size in config).
+    const isCustomEndpoint = baseUrl !== undefined && baseUrl !== '' && !baseUrl.includes('api.anthropic.com');
+    return isCustomEndpoint ? CUSTOM_ENDPOINT_FALLBACK_MAX_TOKENS : FALLBACK_MAX_TOKENS;
   }
   return override === undefined ? ceiling : Math.min(override, ceiling);
 }
@@ -920,7 +931,7 @@ export class AnthropicChatProvider implements ChatProvider {
     this._client = this._apiKey === undefined ? undefined : this._buildClient(this._apiKey);
     this._explicitMaxTokens = options.defaultMaxTokens !== undefined;
     this._generationKwargs = {
-      max_tokens: options.defaultMaxTokens ?? resolveDefaultMaxTokens(options.model),
+      max_tokens: options.defaultMaxTokens ?? resolveDefaultMaxTokens(options.model, undefined, options.baseUrl),
       betaFeatures: options.betaFeatures ?? [INTERLEAVED_THINKING_BETA],
     };
   }
@@ -1270,7 +1281,7 @@ export class AnthropicChatProvider implements ChatProvider {
   }
 
   withMaxCompletionTokens(maxCompletionTokens: number): AnthropicChatProvider {
-    const requestedCap = resolveDefaultMaxTokens(this._model, maxCompletionTokens);
+    const requestedCap = resolveDefaultMaxTokens(this._model, maxCompletionTokens, this._baseUrl);
     const existingCap = this._generationKwargs.max_tokens;
     const clone = this._withGenerationKwargs({
       max_tokens:
diff --git a/packages/kosong/test/anthropic.test.ts b/packages/kosong/test/anthropic.test.ts
index 6b079a75a6..22a8e37d9a 100644
--- a/packages/kosong/test/anthropic.test.ts
+++ b/packages/kosong/test/anthropic.test.ts
@@ -3408,6 +3408,32 @@ describe('resolveDefaultMaxTokens', () => {
     expect(resolveDefaultMaxTokens('vendor-opus-4-7-preview')).toBe(128000);
     expect(resolveDefaultMaxTokens('vendor-opus-4-7-preview', 8000)).toBe(8000);
   });
+
+  it('uses the conservative 32k fallback for a custom endpoint with an unrecognized model', () => {
+    expect(
+      resolveDefaultMaxTokens('some-custom-model', undefined, 'https://ark.cn-beijing.volces.com/api/v3'),
+    ).toBe(32768);
+    expect(resolveDefaultMaxTokens('gpt-5', undefined, 'https://my-proxy.example.test')).toBe(32768);
+  });
+
+  it('keeps the 128k fallback for the official Anthropic endpoint with an unrecognized model', () => {
+    expect(resolveDefaultMaxTokens('gpt-5', undefined, 'https://api.anthropic.com')).toBe(128000);
+    expect(resolveDefaultMaxTokens('totally-unknown-model', undefined, undefined)).toBe(128000);
+  });
+
+  it('still applies the documented ceiling for a Claude model on a custom endpoint', () => {
+    expect(resolveDefaultMaxTokens('claude-opus-4-7', undefined, 'https://my-proxy.example.test')).toBe(
+      128000,
+    );
+    expect(resolveDefaultMaxTokens('claude-3-opus', undefined, 'https://my-proxy.example.test')).toBe(4096);
+  });
+
+  it('honors the user override over the custom-endpoint fallback', () => {
+    expect(resolveDefaultMaxTokens('some-custom-model', 16000, 'https://my-proxy.example.test')).toBe(
+      16000,
+    );
+    expect(resolveDefaultMaxTokens('some-custom-model', 8192, undefined)).toBe(8192);
+  });
 });
 
 describe('AnthropicChatProvider constructor max_tokens', () => {
diff --git a/packages/kosong/test/errors.test.ts b/packages/kosong/test/errors.test.ts
index 2e39e971ed..e02101784d 100644
--- a/packages/kosong/test/errors.test.ts
+++ b/packages/kosong/test/errors.test.ts
@@ -13,6 +13,7 @@ import {
   isRetryableGenerateError,
   isToolExchangeAdjacencyError,
   normalizeAPIStatusError,
+  parseMaxTokensLimit,
 } from '#/errors';
 import { describe, expect, it } from 'vitest';
 
@@ -678,3 +679,40 @@ describe('isImageFormatError', () => {
     ).toBe(false);
   });
 });
+
+describe('parseMaxTokensLimit', () => {
+  it('parses the volcano / ark "expected a value <= N" wording', () => {
+    expect(
+      parseMaxTokensLimit(
+        '400 {"error":{"code":"InvalidParameter","message":"The parameter max_tokens expected a value <= 32768, got 128000."}}',
+      ),
+    ).toBe(32768);
+  });
+
+  it('parses the generic "must be at most N" wording', () => {
+    expect(parseMaxTokensLimit('400 Bad Request: max_tokens must be at most 8192')).toBe(8192);
+  });
+
+  it('parses the generic "cannot exceed N" wording', () => {
+    expect(parseMaxTokensLimit('400 Bad Request: max_tokens cannot exceed 4096')).toBe(4096);
+  });
+
+  it('parses the Anthropic native "max output tokens is N" wording', () => {
+    expect(
+      parseMaxTokensLimit('max_tokens: claude-3-haiku-20240307 max output tokens is 4096'),
+    ).toBe(4096);
+  });
+
+  it('parses the "less than or equal to" and bare "<=" wordings', () => {
+    expect(parseMaxTokensLimit('400 Bad Request: max_tokens must be less than or equal to 4096')).toBe(4096);
+    expect(parseMaxTokensLimit('400 Bad Request: max_tokens <= 2048')).toBe(2048);
+  });
+
+  it('returns null for messages without a max_tokens limit', () => {
+    expect(parseMaxTokensLimit('400 Bad Request: invalid api key')).toBeNull();
+    // A context-overflow message must not be mistaken for a max_tokens ceiling.
+    expect(
+      parseMaxTokensLimit('prompt is too long: the maximum context length is 200000 tokens'),
+    ).toBeNull();
+  });
+});