diff --git a/src/agents/embedded-agent-runner/model.test.ts b/src/agents/embedded-agent-runner/model.test.ts
index 42e054ddbb12..f19691768ad0 100644
--- a/src/agents/embedded-agent-runner/model.test.ts
+++ b/src/agents/embedded-agent-runner/model.test.ts
@@ -1243,6 +1243,39 @@ describe("resolveModel", () => {
     });
   });
 
+  it("leaves maxTokens undefined when no configured or catalog value is available (regression: #98295)", () => {
+    // Regression for https://github.com/openclaw/openclaw/issues/98295.
+    // A custom provider entry without maxTokens (and no matching bundled
+    // static catalog row) must not synthesize an oversized output cap from
+    // DEFAULT_CONTEXT_TOKENS. Leaving maxTokens undefined lets the transport
+    // omit `max_completion_tokens` so the provider applies its own default,
+    // avoiding HTTP 400 (Param Incorrect) from strict OpenAI-compatible
+    // servers whose completion-token ceiling is below the synthesized value.
+    resolveBundledStaticCatalogModelMock.mockReturnValueOnce(undefined);
+    const cfg = {
+      models: {
+        providers: {
+          xiaomi: {
+            baseUrl: "https://api.xiaomimimo.com/v1",
+            models: [
+              {
+                id: "mimo-v2.5-pro",
+                name: "mimo-v2.5-pro",
+              },
+            ],
+          },
+        },
+      },
+    } as unknown as OpenClawConfig;
+
+    const result = resolveModelForTest("xiaomi", "mimo-v2.5-pro", "/tmp/agent", cfg);
+    const model = expectResolvedModel(result);
+
+    expect(model.id).toBe("mimo-v2.5-pro");
+    expect(model.baseUrl).toBe("https://api.xiaomimimo.com/v1");
+    expect(model.maxTokens).toBeUndefined();
+  });
+
   it("inherits bundled static transport for configured provider fallback models", () => {
     resolveBundledStaticCatalogModelMock.mockReturnValueOnce({
       provider: "deepseek",
diff --git a/src/agents/embedded-agent-runner/model.ts b/src/agents/embedded-agent-runner/model.ts
index 9ea0fb05540c..723ac2e2517c 100644
--- a/src/agents/embedded-agent-runner/model.ts
+++ b/src/agents/embedded-agent-runner/model.ts
@@ -1330,6 +1330,11 @@ function resolveConfiguredFallbackModel(params: {
     compat: fallbackCompat,
     reasoning: metadataModel?.reasoning,
   });
+  const resolvedFallbackMaxTokens =
+    configuredModel?.maxTokens ??
+    providerConfig?.maxTokens ??
+    providerConfig?.models?.[0]?.maxTokens ??
+    staticCatalogModel?.maxTokens;
   return normalizeResolvedModel({
     provider,
     cfg,
@@ -1365,12 +1370,11 @@ function resolveConfiguredFallbackModel(params: {
             providerConfig?.contextTokens ??
             providerConfig?.models?.[0]?.contextTokens ??
             staticCatalogModel?.contextTokens,
-          maxTokens:
-            configuredModel?.maxTokens ??
-            providerConfig?.maxTokens ??
-            providerConfig?.models?.[0]?.maxTokens ??
-            staticCatalogModel?.maxTokens ??
-            DEFAULT_CONTEXT_TOKENS,
+          // maxTokens is a wire-level output cap, not a context-budget fallback.
+          // Omit an unknown cap so strict providers can apply their own limit.
+          ...(resolvedFallbackMaxTokens !== undefined
+            ? { maxTokens: resolvedFallbackMaxTokens }
+            : {}),
           ...(resolvedParams ? { params: resolvedParams } : {}),
           ...(requestTimeoutMs !== undefined ? { requestTimeoutMs } : {}),
           headers: requestConfig.headers,
diff --git a/src/agents/openai-transport-stream.test.ts b/src/agents/openai-transport-stream.test.ts
index be45f61c18af..9ed5047c0d43 100644
--- a/src/agents/openai-transport-stream.test.ts
+++ b/src/agents/openai-transport-stream.test.ts
@@ -7667,6 +7667,31 @@ describe("openai transport stream", () => {
     expect(params).not.toHaveProperty("max_tokens");
   });
 
+  it("omits output-token fields when the resolved model has no known cap", () => {
+    const params = buildOpenAICompletionsParams(
+      {
+        id: "mimo-v2.5-pro",
+        name: "MiMo V2.5 Pro",
+        api: "openai-completions",
+        provider: "xiaomi",
+        baseUrl: "https://api.xiaomimimo.com/v1",
+        reasoning: true,
+        input: ["text"],
+        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+        contextWindow: 1_048_576,
+      } as Model<"openai-completions">,
+      {
+        systemPrompt: "system",
+        messages: [],
+        tools: [],
+      } as never,
+      undefined,
+    );
+
+    expect(params).not.toHaveProperty("max_completion_tokens");
+    expect(params).not.toHaveProperty("max_tokens");
+  });
+
   it("uses model params max_completion_tokens for OpenAI completions before model maxTokens", () => {
     const params = buildOpenAICompletionsParams(
       {