diff --git a/src/server/__tests__/providers.test.ts b/src/server/__tests__/providers.test.ts
index 7cd651c6..2a5f01e8 100644
--- a/src/server/__tests__/providers.test.ts
+++ b/src/server/__tests__/providers.test.ts
@@ -137,6 +137,33 @@ describe('ProviderService', () => {
       await expect(fs.readFile(path.join(tmpDir, 'cc-haha', 'settings.json'), 'utf-8')).rejects.toThrow()
     })
 
+    test('custom providers declare thinking and effort capability passthrough for user-defined models', async () => {
+      const svc = new ProviderService()
+      const provider = await svc.addProvider(sampleInput({
+        models: {
+          main: 'deepseek-ai/DeepSeek-V4-Pro',
+          haiku: 'deepseek-ai/DeepSeek-V4-Pro',
+          sonnet: 'deepseek-ai/DeepSeek-V4-Pro',
+          opus: 'deepseek-ai/DeepSeek-V4-Pro',
+        },
+      }))
+
+      await svc.activateProvider(provider.id)
+
+      const settings = await readSettings()
+      const env = settings.env as Record<string, string>
+      expect(env.ANTHROPIC_DEFAULT_SONNET_MODEL).toBe('deepseek-ai/DeepSeek-V4-Pro')
+      expect(env.ANTHROPIC_DEFAULT_SONNET_MODEL_SUPPORTED_CAPABILITIES).toBe(
+        'thinking,effort,adaptive_thinking,max_effort',
+      )
+      expect(env.ANTHROPIC_DEFAULT_HAIKU_MODEL_SUPPORTED_CAPABILITIES).toBe(
+        'thinking,effort,adaptive_thinking,max_effort',
+      )
+      expect(env.ANTHROPIC_DEFAULT_OPUS_MODEL_SUPPORTED_CAPABILITIES).toBe(
+        'thinking,effort,adaptive_thinking,max_effort',
+      )
+    })
+
     test('adding additional providers should keep activeId unchanged', async () => {
       const svc = new ProviderService()
       await svc.addProvider(sampleInput({ name: 'First' }))
diff --git a/src/server/services/providerService.ts b/src/server/services/providerService.ts
index fb5739a3..f0d55f33 100644
--- a/src/server/services/providerService.ts
+++ b/src/server/services/providerService.ts
@@ -44,6 +44,8 @@ const MANAGED_ENV_KEYS = [
   MODEL_CONTEXT_WINDOWS_ENV_KEY,
 ] as const
 
+const CUSTOM_PROVIDER_MODEL_CAPABILITIES = 'thinking,effort,adaptive_thinking,max_effort'
+
 const DEFAULT_INDEX: ProvidersIndex = { activeId: null, providers: [] }
 const AUTH_ENV_KEYS = new Set(['ANTHROPIC_API_KEY', 'ANTHROPIC_AUTH_TOKEN'])
 
@@ -317,9 +319,18 @@ export class ProviderService {
     }
 
     const presetDefaultEnv = getPresetDefaultEnv(provider.presetId)
+    const customProviderCapabilityEnv =
+      provider.presetId === 'custom'
+        ? {
+            ANTHROPIC_DEFAULT_HAIKU_MODEL_SUPPORTED_CAPABILITIES: CUSTOM_PROVIDER_MODEL_CAPABILITIES,
+            ANTHROPIC_DEFAULT_SONNET_MODEL_SUPPORTED_CAPABILITIES: CUSTOM_PROVIDER_MODEL_CAPABILITIES,
+            ANTHROPIC_DEFAULT_OPUS_MODEL_SUPPORTED_CAPABILITIES: CUSTOM_PROVIDER_MODEL_CAPABILITIES,
+          }
+        : {}
 
     return {
       ...omitAuthEnv(presetDefaultEnv),
+      ...customProviderCapabilityEnv,
       ...(provider.autoCompactWindow !== undefined && {
         CLAUDE_CODE_AUTO_COMPACT_WINDOW: String(provider.autoCompactWindow),
       }),