diff --git a/open-sse/translator/request/openai-to-claude.ts b/open-sse/translator/request/openai-to-claude.ts
index 50f2973bb7..8e7262c453 100644
--- a/open-sse/translator/request/openai-to-claude.ts
+++ b/open-sse/translator/request/openai-to-claude.ts
@@ -5,12 +5,93 @@ import { 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";
+import { capMaxOutputTokens } from "../../../src/lib/modelCapabilities.ts";
 
 // Prefix for Claude OAuth tool names to avoid conflicts
 // Can be disabled per-request via body._disableToolPrefix = true
 export const CLAUDE_OAUTH_TOOL_PREFIX = "proxy_";
 const CLAUDE_TOOL_CHOICE_REQUIRED = "an" + "y";
 
+// Anthropic constraints for the thinking + max_tokens contract:
+//   - thinking.budget_tokens must be >= 1024 when thinking is enabled
+//   - max_tokens must be > thinking.budget_tokens (covers thinking + response)
+//   - max_tokens must be <= model output cap (e.g. 128000 for Opus 4.7)
+const MIN_CLAUDE_THINKING_BUDGET = 1024;
+const MIN_RESPONSE_ROOM = 1024;
+const FALLBACK_OUTPUT_CAP = 128000;
+
+function safeCapMaxOutputTokens(model: string): number {
+  try {
+    const cap = capMaxOutputTokens(model);
+    return typeof cap === "number" && cap > 0 ? cap : FALLBACK_OUTPUT_CAP;
+  } catch {
+    return FALLBACK_OUTPUT_CAP;
+  }
+}
+
+/**
+ * Fit Claude thinking budget within the model's max output cap.
+ *
+ * Replaces the previous unconditional `max_tokens = budget + 8192` inflation,
+ * which could exceed the model output cap (e.g. Opus 4.7's 128000 ceiling) and
+ * trigger HTTP 400 from Anthropic ("max_tokens > 128000").
+ *
+ * Strategy (preserves caller intent up to the model cap):
+ *   - Preserve caller's max_tokens as response room (floored to MIN_RESPONSE_ROOM)
+ *   - Target max_tokens = responseRoom + requestedBudget, capped at modelCap
+ *   - fittedBudget = max_tokens - responseRoom (the thinking budget actually used)
+ *   - If the cap squeezes fittedBudget below the Anthropic minimum, retry with
+ *     responseRoom shrunk to MIN_RESPONSE_ROOM; if still below MIN, disable
+ *     thinking entirely (cap too tight for any reasoning).
+ *
+ * Worked example (real-world Opus 4.7 case that previously 400'd):
+ *   caller max_tokens = 32000, reasoning_effort=high → budget = 131072,
+ *   model cap = 128000.
+ *   responseRoom = max(32000, 1024) = 32000
+ *   target       = min(32000 + 131072, 128000) = 128000
+ *   fittedBudget = 128000 - 32000 = 96000  (>= 1024, OK)
+ *   → max_tokens=128000, budget_tokens=96000 (vs. the old buggy 139264 / 131072).
+ */
+export function fitThinkingToMaxTokens(
+  model: string,
+  callerMaxTokens: number,
+  thinking: Record<string, unknown> | undefined
+): { maxTokens: number; thinking: Record<string, unknown> | undefined } {
+  const modelCap = safeCapMaxOutputTokens(model);
+  const requestedBudget = Number(thinking?.budget_tokens) || 0;
+
+  // No budgeted thinking — just cap max_tokens to the model output ceiling.
+  if (!thinking || requestedBudget <= 0) {
+    return {
+      maxTokens: Math.min(Math.max(callerMaxTokens, 1), modelCap),
+      thinking,
+    };
+  }
+
+  let responseRoom = Math.max(callerMaxTokens, MIN_RESPONSE_ROOM);
+  let target = Math.min(responseRoom + requestedBudget, modelCap);
+  let fittedBudget = target - responseRoom;
+
+  // If the cap squeezed thinking below Anthropic's floor, try shrinking
+  // response room to MIN_RESPONSE_ROOM to recover budget.
+  if (fittedBudget < MIN_CLAUDE_THINKING_BUDGET && responseRoom > MIN_RESPONSE_ROOM) {
+    responseRoom = MIN_RESPONSE_ROOM;
+    target = Math.min(responseRoom + requestedBudget, modelCap);
+    fittedBudget = target - responseRoom;
+  }
+
+  // Cap too tight for any thinking — disable rather than send an invalid request.
+  if (fittedBudget < MIN_CLAUDE_THINKING_BUDGET) {
+    return { maxTokens: modelCap, thinking: undefined };
+  }
+
+  const adjustedThinking: Record<string, unknown> = { ...thinking };
+  if (fittedBudget < requestedBudget) {
+    adjustedThinking.budget_tokens = fittedBudget;
+  }
+  return { maxTokens: target, thinking: adjustedThinking };
+}
+
 type ClaudeContentBlock = Record<string, unknown>;
 type ClaudeMessage = {
   role: string;
@@ -374,18 +455,21 @@ export function openaiToClaudeRequest(model, body, stream) {
           type: "enabled",
           budget_tokens: budget,
         };
-        // Claude requires max_tokens > budget_tokens
-        if (result.max_tokens <= budget) {
-          result.max_tokens = budget + 8192;
-        }
       }
     }
   }
 
-  // Ensure max_tokens > budget_tokens for all thinking configurations (#627)
-  const budgetTokens = Number(result.thinking?.budget_tokens) || 0;
-  if (budgetTokens > 0 && result.max_tokens <= budgetTokens) {
-    result.max_tokens = budgetTokens + 8192;
+  // Fit thinking budget within the model's output cap and ensure
+  // max_tokens > budget_tokens for all thinking configurations (#627).
+  // Replaces the previous unconditional `budget + 8192` inflation, which
+  // could exceed model caps (e.g. Opus 4.7's 128000 ceiling) and trigger
+  // HTTP 400 from Anthropic.
+  const fitted = fitThinkingToMaxTokens(model, Number(result.max_tokens) || 0, result.thinking);
+  result.max_tokens = fitted.maxTokens;
+  if (fitted.thinking === undefined) {
+    delete result.thinking;
+  } else {
+    result.thinking = fitted.thinking;
   }
 
   // Attach toolNameMap to result for response translation
diff --git a/tests/unit/translator-openai-to-claude.test.ts b/tests/unit/translator-openai-to-claude.test.ts
index 2c6b63a0f3..c7d77f69ab 100644
--- a/tests/unit/translator-openai-to-claude.test.ts
+++ b/tests/unit/translator-openai-to-claude.test.ts
@@ -237,6 +237,9 @@ test("OpenAI -> Claude maps tool_choice and injects response_format instructions
 });
 
 test("OpenAI -> Claude turns reasoning settings into thinking budgets and expands max tokens", () => {
+  // `claude-4-sonnet` is a fixture that doesn't match any spec → default cap = 8192.
+  // fitThinkingToMaxTokens floors response room at MIN_RESPONSE_ROOM (1024)
+  // and targets max_tokens = responseRoom + budget capped at modelCap.
   const effortResult = openaiToClaudeRequest(
     "claude-4-sonnet",
     {
@@ -248,7 +251,8 @@ test("OpenAI -> Claude turns reasoning settings into thinking budgets and expand
   );
 
   assert.deepEqual(effortResult.thinking, { type: "enabled", budget_tokens: 1024 });
-  assert.equal(effortResult.max_tokens, 9216);
+  // responseRoom=max(10,1024)=1024; target=min(1024+1024, 8192)=2048
+  assert.equal(effortResult.max_tokens, 2048);
 
   const explicitThinkingResult = openaiToClaudeRequest(
     "claude-4-sonnet",
@@ -265,7 +269,8 @@ test("OpenAI -> Claude turns reasoning settings into thinking budgets and expand
     budget_tokens: 2000,
     max_tokens: 3000,
   });
-  assert.equal(explicitThinkingResult.max_tokens, 10192);
+  // responseRoom=max(1000,1024)=1024; target=min(1024+2000, 8192)=3024
+  assert.equal(explicitThinkingResult.max_tokens, 3024);
 });
 
 test("OpenAI -> Claude preserves xhigh only for Claude models that expose it", () => {
@@ -290,9 +295,40 @@ test("OpenAI -> Claude preserves xhigh only for Claude models that expose it", (
 
   assert.deepEqual(preserved.thinking, { type: "adaptive" });
   assert.deepEqual(preserved.output_config, { effort: "xhigh" });
-  assert.deepEqual(downgraded.thinking, { type: "enabled", budget_tokens: 131072 });
+  // standardModel (claude-opus-4-6) has output cap 128000.
+  // Requested budget 131072 is cap-fitted: target=min(1024+131072, 128000)=128000;
+  // fittedBudget=128000-1024=126976. budget shrinks to fit within model cap
+  // rather than producing invalid max_tokens=139264 that Anthropic rejects with 400.
+  assert.deepEqual(downgraded.thinking, { type: "enabled", budget_tokens: 126976 });
   assert.equal(downgraded.output_config, undefined);
-  assert.equal(downgraded.max_tokens, 139264);
+  assert.equal(downgraded.max_tokens, 128000);
+});
+
+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
+  // `budget + 8192 = 139264` exceeded Opus 4.7's 128000 output cap and caused
+  // HTTP 400 "max_tokens > 128000".
+  // fitThinkingToMaxTokens must preserve caller's 32000 response room and
+  // shrink budget to (128000 - 32000) = 96000.
+  const result = openaiToClaudeRequest(
+    "claude-opus-4-7",
+    {
+      messages: [{ role: "user", content: "Reason about something hard" }],
+      max_tokens: 32000,
+      reasoning_effort: "high",
+    },
+    false
+  );
+
+  assert.equal(result.max_tokens, 128000, "max_tokens must equal model cap, not 139264");
+  assert.ok(result.thinking, "thinking should remain enabled");
+  assert.equal((result.thinking as { type: string }).type, "enabled");
+  assert.equal(
+    (result.thinking as { budget_tokens: number }).budget_tokens,
+    96000,
+    "budget must shrink to (cap - caller max_tokens) to preserve response room"
+  );
 });
 
 test("OpenAI -> Claude can disable OAuth prefixes and Antigravity strips Claude-only prompting", () => {