fix(claude): reconcile max_tokens vs thinking budget and lift per-model ceiling (#2381)

On the translated OpenAI->Claude path, adjustMaxTokens capped max_tokens
before applyThinking set thinking.budget_tokens, so max-effort budget
(128000) could exceed a 64k-clamped max_tokens -> Anthropic 400.
prepareClaudeRequest now reconciles after the budget is known: prefer
raising max_tokens, only shrink budget when it meets/exceeds the ceiling.

Also lift the global 64000 cap: the ceiling is now the model's real
maxOutput, so high-output models (fable/mythos, opus-4.8/sonnet-4.6) get
their full budget. adjustMaxTokens gains an optional ceiling arg (default
unchanged, callers untouched); openai-to-claude passes the model maxOutput.

Native Claude Code passthrough is unaffected.

Co-Authored-By: Claude <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/translator/formats/claude.js

@@ -192,10 +192,27 @@ export function prepareClaudeRequest(body, provider = null, apiKey = null, conne
     delete body.output_config;
   }
 
-  // Clamp max_tokens to the model output ceiling (never above DEFAULT_MAX_TOKENS)
+  // Clamp max_tokens to the model's real output ceiling. Models whose caps
+  // declare a higher maxOutput (e.g. Opus 4.8 / Sonnet 4.6 = 128000) are allowed
+  // up to it, so max-effort thinking gets full budget; others fall back to the
+  // conservative 64000 default.
   if (body.max_tokens) {
-    const ceiling = Math.min(getCapabilitiesForModel(provider, body.model).maxOutput, DEFAULT_MAX_TOKENS);
+    const ceiling = getCapabilitiesForModel(provider, body.model).maxOutput || DEFAULT_MAX_TOKENS;
     if (body.max_tokens > ceiling) body.max_tokens = ceiling;
+
+    // Reconcile against thinking budget. applyThinking (thinkingUnified.js) runs
+    // AFTER adjustMaxTokens capped max_tokens, and the claude-budget format maps
+    // max effort → budget_tokens 128000 — larger than the clamped max_tokens.
+    // Anthropic requires max_tokens strictly greater than budget_tokens (else 400).
+    // Prefer raising max_tokens to preserve the requested thinking depth; if the
+    // budget alone meets/exceeds the ceiling, cap output and shrink the budget so
+    // some tokens remain for the answer.
+    if (body.thinking?.type === "enabled" && body.thinking.budget_tokens && body.thinking.budget_tokens >= body.max_tokens) {
+      body.max_tokens = Math.min(body.thinking.budget_tokens + 1024, ceiling);
+      if (body.thinking.budget_tokens >= body.max_tokens) {
+        body.thinking.budget_tokens = Math.max(1024, body.max_tokens - 1024);
+      }
+    }
   }
 
   // 1. System: remove all cache_control, add only to last block with ttl 1h

open-sse/translator/formats/maxTokens.js

@@ -3,9 +3,13 @@ import { DEFAULT_MAX_TOKENS, DEFAULT_MIN_TOKENS } from "../../config/runtimeConf
 /**
  * Adjust max_tokens based on request context
  * @param {object} body - Request body
+ * @param {number} [ceiling=DEFAULT_MAX_TOKENS] - Upper bound for max_tokens.
+ *   Callers with model context (e.g. openai-to-claude) pass the model's real
+ *   maxOutput so high-output models (Opus 4.8 = 128000) aren't pre-clamped to
+ *   the conservative 64000 default before the model-aware step sees them.
  * @returns {number} Adjusted max_tokens
  */
-export function adjustMaxTokens(body) {
+export function adjustMaxTokens(body, ceiling = DEFAULT_MAX_TOKENS) {
   let maxTokens = body.max_tokens || DEFAULT_MAX_TOKENS;
 
   // Auto-increase for tool calling to prevent truncated arguments (min never above max)
@@ -16,14 +20,14 @@ export function adjustMaxTokens(body) {
   }
 
   // Ensure max_tokens > thinking.budget_tokens (Claude API requirement)
-  // Claude API requires strictly greater, so add buffer instead of using DEFAULT_MAX_TOKENS
-  // which could equal budget_tokens when budget_tokens >= 64000
+  // Claude API requires strictly greater, so add buffer instead of using the
+  // ceiling which could equal budget_tokens when budget_tokens >= ceiling
   if (body.thinking?.budget_tokens && maxTokens <= body.thinking.budget_tokens) {
     maxTokens = body.thinking.budget_tokens + 1024;
   }
 
-  // Never exceed the global ceiling
-  if (maxTokens > DEFAULT_MAX_TOKENS) maxTokens = DEFAULT_MAX_TOKENS;
+  // Never exceed the ceiling
+  if (maxTokens > ceiling) maxTokens = ceiling;
 
   return maxTokens;
 }

open-sse/translator/request/openai-to-claude.js

@@ -6,6 +6,7 @@ import { safeParseJSON } from "../concerns/json.js";
 import { parseDataUri } from "../concerns/image.js";
 import { extractTextContent } from "../formats/gemini.js";
 import { ROLE, OPENAI_BLOCK, CLAUDE_BLOCK } from "../schema/index.js";
+import { getCapabilitiesForModel } from "../../providers/capabilities.js";
 
 // Empty prefix matches real Claude Code behavior (no tool name prefix).
 // Previously "proxy_" was used but this is a detectable fingerprint difference.
@@ -15,9 +16,13 @@ const CLAUDE_OAUTH_TOOL_PREFIX = "";
 export function openaiToClaudeRequest(model, body, stream) {
   // Tool name mapping for Claude OAuth (capitalizedName → originalName)
   const toolNameMap = new Map();
+  // Cap max_tokens at the model's real output ceiling (e.g. Opus 4.8 = 128000),
+  // not the conservative 64000 default — otherwise a high-output model is
+  // pre-clamped here before prepareClaudeRequest's model-aware step runs.
+  const modelCeiling = getCapabilitiesForModel(null, model).maxOutput || undefined;
   const result = {
     model: model,
-    max_tokens: adjustMaxTokens(body),
+    max_tokens: adjustMaxTokens(body, modelCeiling),
     stream: stream
   };
 

tests/translator/bugs-toClaude-context.test.js

@@ -67,6 +67,101 @@ describe("OpenAI → Claude context mapping", () => {
     expect(JSON.stringify(out), "remote image dropped").toContain("pic.png");
   });
 
+  // prepareClaudeRequest reconciles max_tokens vs thinking.budget_tokens.
+  // applyThinking runs after adjustMaxTokens caps max_tokens, so a claude-budget
+  // model at "max" effort (budget 128000) can exceed the clamped max_tokens and
+  // trip Anthropic's "max_tokens > budget_tokens" rule (400). See claude.js.
+  describe("max_tokens vs thinking.budget_tokens reconciliation", () => {
+    // 64k-ceiling model (maxOutput 64000) + max-effort budget 128000: budget alone
+    // exceeds the ceiling → cap max_tokens at 64000 and shrink budget below it.
+    it("max effort budget on a 64k model → budget < max_tokens ≤ 64000", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-opus-4-20250514",
+        max_tokens: 64000,
+        thinking: { type: "enabled", budget_tokens: 128000 },
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.max_tokens).toBe(64000);
+      expect(out.thinking.budget_tokens).toBeLessThan(out.max_tokens);
+      expect(out.thinking.budget_tokens).toBeGreaterThan(0);
+    });
+
+    // Budget fits under the ceiling but exceeds a small client max_tokens →
+    // raise max_tokens to fit, preserving the requested thinking depth.
+    it("xhigh budget with a low client max_tokens → raise max_tokens, preserve budget", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-opus-4-20250514",
+        max_tokens: 16000,
+        thinking: { type: "enabled", budget_tokens: 32768 },
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.thinking.budget_tokens).toBe(32768);
+      expect(out.max_tokens).toBe(33792); // 32768 + 1024, under the 64000 ceiling
+    });
+
+    // Budget already below max_tokens → nothing to reconcile.
+    it("high budget under max_tokens → both unchanged", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-opus-4-20250514",
+        max_tokens: 64000,
+        thinking: { type: "enabled", budget_tokens: 24576 },
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.max_tokens).toBe(64000);
+      expect(out.thinking.budget_tokens).toBe(24576);
+    });
+
+    // Non-budget thinking shapes (adaptive / disabled) carry no budget_tokens →
+    // the reconciliation must never touch them.
+    it("adaptive thinking (no budget_tokens) is left untouched", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-opus-4-20250514",
+        max_tokens: 64000,
+        thinking: { type: "adaptive" },
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.max_tokens).toBe(64000);
+      expect(out.thinking).toEqual({ type: "adaptive" });
+    });
+
+    // Lifted ceiling: a claude-budget model whose caps declare maxOutput 128000
+    // (e.g. fable) may use the full budget at max effort instead of being pinned
+    // to the conservative 64000 default.
+    it("max effort budget on a 128k model → max_tokens up to 128000, budget preserved just under", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-fable-5",
+        max_tokens: 64000,
+        thinking: { type: "enabled", budget_tokens: 128000 },
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.max_tokens).toBe(128000);
+      expect(out.thinking.budget_tokens).toBe(126976); // 128000 - 1024
+      expect(out.thinking.budget_tokens).toBeLessThan(out.max_tokens);
+    });
+
+    // Regression: a default 64k-ceiling model still clamps an over-large client
+    // max_tokens down to 64000 (the lift is per-model, not global).
+    it("over-large client max_tokens on a 64k model is still clamped to 64000", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-opus-4-20250514",
+        max_tokens: 120000,
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.max_tokens).toBe(64000);
+    });
+
+    // Lifted ceiling for a 128k model: a large client max_tokens is now allowed
+    // through instead of being clamped to 64000.
+    it("large client max_tokens on a 128k model is allowed up to maxOutput", () => {
+      const out = prepareClaudeRequest({
+        model: "claude-fable-5",
+        max_tokens: 100000,
+        messages: [{ role: "user", content: "q" }],
+      }, "anthropic");
+      expect(out.max_tokens).toBe(100000);
+    });
+  });
+
   it("DeepSeek Claude transport adds a thinking placeholder before tool_use in thinking mode", () => {
     const out = prepareClaudeRequest({
       model: "deepseek-v4-pro",