diff --git a/packages/local-runtime-v2/src/service/model-system/resolution/local-model-resolver.test.ts b/packages/local-runtime-v2/src/service/model-system/resolution/local-model-resolver.test.ts
index a9124e94a..b8e4c32f3 100644
--- a/packages/local-runtime-v2/src/service/model-system/resolution/local-model-resolver.test.ts
+++ b/packages/local-runtime-v2/src/service/model-system/resolution/local-model-resolver.test.ts
@@ -1447,6 +1447,10 @@ describe('LocalModelResolver custom provider compat overrides', () => {
     expect(await systemPromptRoleFor(undefined)).toBe('developer');
   });
 
+  it('keeps the system role for a thinking model on the Mistral API', async () => {
+    expect(await systemPromptRoleFor(undefined, 'https://api.mistral.ai/v1')).toBe('system');
+  });
+
   it.each([
     'https://api.siliconflow.cn/v1',
     'https://api.siliconflow.com/v1',
@@ -1474,6 +1478,8 @@ describe('LocalModelResolver custom provider compat overrides', () => {
   it.each([
     ['https://api.openai.com/v1', 'developer'],
     ['https://api.deepseek.com/v1', 'system'],
+    ['https://api.mistral.ai.example/v1', 'developer'],
+    ['https://gateway.example/api.mistral.ai/v1', 'developer'],
     ['https://api.siliconflow.cn.example/v1', 'developer'],
     ['https://gateway.example/api.siliconflow.cn/v1', 'developer'],
     ['https://api.siliconflow.com.example/v1', 'developer'],
@@ -1493,6 +1499,12 @@ describe('LocalModelResolver custom provider compat overrides', () => {
     expect(await systemPromptRoleFor(undefined, baseURL)).toBe(role);
   });
 
+  it('honors an explicit developer-role override on the Mistral API', async () => {
+    expect(
+      await systemPromptRoleFor({ supportsDeveloperRole: true }, 'https://api.mistral.ai/v1'),
+    ).toBe('developer');
+  });
+
   it.each([
     'https://api.siliconflow.cn/v1',
     'https://api.siliconflow.com/v1',
diff --git a/third_party/pi-mono/MINIMAX_CHANGES.md b/third_party/pi-mono/MINIMAX_CHANGES.md
index 10dd06116..c9057b542 100644
--- a/third_party/pi-mono/MINIMAX_CHANGES.md
+++ b/third_party/pi-mono/MINIMAX_CHANGES.md
@@ -13,6 +13,14 @@ This directory vendors `pi-mono` as source so MiniMax can patch, validate, and s
 
 No upstream source files are changed in the baseline import.
 
+### 2026-09-19 — preserve the system role for Mistral Chat Completions
+
+- Reason: thinking-enabled custom OpenAI-compatible connections to `api.mistral.ai` emitted `developer`, which is absent from the [Mistral Chat Completions message contract](https://docs.mistral.ai/api/endpoint/chat). [OpenClaw's compatibility defaults](https://github.com/openclaw/openclaw/blob/e2bcb1614de060927121bd72de850cee3a08d308/packages/ai/src/transports/openai-completions-compat.ts#L184-L210) also disable this role for the Mistral public endpoint.
+- Affected package: `packages/ai` (`@earendil-works/pi-ai`), OpenAI Completions compatibility detection.
+- Change: default to `system` only on the exact Mistral API host. Preserve thinking, explicit compatibility overrides, all other request options, and other endpoints. Native Mistral transport is unchanged.
+- Upstream PR: not opened.
+- Validation: outgoing-payload regressions in `packages/local-runtime-v2/src/service/model-system/resolution/local-model-resolver.test.ts`; the endpoint regression fails before the fix. No live Mistral requests or full CLI acceptance were performed; the decision is based on the official contract and current upstream implementation.
+
 ### 2026-09-19 — preserve the system role for SiliconFlow
 
 - Reason: thinking-enabled OpenAI-compatible requests sent the `developer` role, which is absent from SiliconFlow's documented message schema.
diff --git a/third_party/pi-mono/packages/ai/src/providers/openai-completions.ts b/third_party/pi-mono/packages/ai/src/providers/openai-completions.ts
index 2c08bf0cf..4d7b2d0fa 100644
--- a/third_party/pi-mono/packages/ai/src/providers/openai-completions.ts
+++ b/third_party/pi-mono/packages/ai/src/providers/openai-completions.ts
@@ -1095,6 +1095,7 @@ function detectCompat(model: Model<"openai-completions">): ResolvedOpenAIComplet
 		/^https?:\/\/(?:dashscope(?:-intl|-us)?|(?:coding(?:-intl)?|cn-hongkong)\.dashscope)\.aliyuncs\.com(?::\d+)?(?:\/|$)/i.test(baseUrl) ||
 		/^https?:\/\/[a-z0-9-]+\.(?:cn-beijing|cn-hongkong|ap-southeast-1|ap-northeast-1|eu-central-1|us-east-1)\.maas\.aliyuncs\.com(?::\d+)?(?:\/|$)/i.test(baseUrl);
 	const isSiliconFlow = /^https?:\/\/api\.siliconflow\.(?:cn|com)(?::\d+)?(?:\/|$)/i.test(baseUrl);
+	const isMistralApi = /^https?:\/\/api\.mistral\.ai(?::\d+)?(?:\/|$)/i.test(baseUrl);
 	const isOpenRouter = provider === "openrouter" || baseUrl.includes("openrouter.ai");
 	const isCloudflareWorkersAI = provider === "cloudflare-workers-ai" || baseUrl.includes("api.cloudflare.com");
 	const isCloudflareAiGateway = provider === "cloudflare-ai-gateway" || baseUrl.includes("gateway.ai.cloudflare.com");
@@ -1130,7 +1131,11 @@ function detectCompat(model: Model<"openai-completions">): ResolvedOpenAIComplet
 	return {
 		supportsStore: !isNonStandard,
 		supportsDeveloperRole:
-			!isKimiCoding && !isDashScope && !isSiliconFlow && (isOpenRouterDeveloperRoleModel || (!isNonStandard && !isOpenRouter)),
+			!isKimiCoding &&
+			!isDashScope &&
+			!isSiliconFlow &&
+			!isMistralApi &&
+			(isOpenRouterDeveloperRoleModel || (!isNonStandard && !isOpenRouter)),
 		supportsReasoningEffort:
 			!isGrok && !isZai && !isMoonshot && !isTogether && !isCloudflareAiGateway && !isNvidia && !isAntLing,
 		supportsUsageInStreaming: true,