diff --git a/src/services/api/grok/index.ts b/src/services/api/grok/index.ts
index bb0ead96c4..55c91dad4a 100644
--- a/src/services/api/grok/index.ts
+++ b/src/services/api/grok/index.ts
@@ -12,6 +12,7 @@ import type {
   ChatCompletionCreateParamsStreaming,
 } from 'openai/resources/chat/completions/completions.mjs'
 import { getGrokClient } from './client.js'
+import { updateOpenAIUsage } from '../openai/openaiShared.js'
 import {
   anthropicMessagesToOpenAI,
   anthropicToolsToOpenAI,
@@ -136,7 +137,7 @@ export async function* queryModelGrok(
           partialMessage = (event as any).message
           ttftMs = Date.now() - start
           if ((event as any).message?.usage) {
-            usage = { ...usage, ...(event as any).message.usage }
+            usage = updateOpenAIUsage(usage, (event as any).message.usage)
           }
           break
         }
@@ -192,7 +193,7 @@ export async function* queryModelGrok(
         case 'message_delta': {
           const deltaUsage = (event as any).usage
           if (deltaUsage) {
-            usage = { ...usage, ...deltaUsage }
+            usage = updateOpenAIUsage(usage, deltaUsage)
           }
           break
         }
diff --git a/src/services/api/openai/index.ts b/src/services/api/openai/index.ts
index 520290b189..b76be9b9b7 100644
--- a/src/services/api/openai/index.ts
+++ b/src/services/api/openai/index.ts
@@ -10,6 +10,7 @@ import type {
 import type { AgentId } from '../../../types/ids.js'
 import type { Tools } from '../../../Tool.js'
 import { getOpenAIClient } from './client.js'
+import { updateOpenAIUsage } from './openaiShared.js'
 import {
   anthropicMessagesToOpenAI,
   resolveOpenAIModel,
@@ -449,7 +450,7 @@ export async function* queryModelOpenAI(
         case 'message_delta': {
           const deltaUsage = (event as any).usage
           if (deltaUsage) {
-            usage = { ...usage, ...deltaUsage }
+            usage = updateOpenAIUsage(usage, deltaUsage)
           }
           if ((event as any).delta?.stop_reason != null) {
             stopReason = (event as any).delta.stop_reason
diff --git a/src/services/api/openai/openaiShared.ts b/src/services/api/openai/openaiShared.ts
new file mode 100644
index 0000000000..6012080a6e
--- /dev/null
+++ b/src/services/api/openai/openaiShared.ts
@@ -0,0 +1,46 @@
+/**
+ * Shared utilities for OpenAI-compatible API paths.
+ *
+ * Both the OpenAI path (queryModelOpenAI) and Grok path (queryModelGrok) use
+ * the same adapters (openaiStreamAdapter, openaiConvertMessages), so the event
+ * processing logic should be shared rather than duplicated.
+ */
+
+/**
+ * Merge a delta usage into the accumulated usage, preserving cache-related
+ * fields from previous values when the delta carries explicit zeroes or
+ * undefined values.
+ *
+ * Mirrors updateUsage() in claude.ts: a future adapter change that omits
+ * cache fields from certain streaming events should not silently zero the
+ * accumulated counters.
+ */
+export function updateOpenAIUsage(
+  current: {
+    input_tokens: number
+    output_tokens: number
+    cache_creation_input_tokens: number
+    cache_read_input_tokens: number
+  },
+  delta: {
+    input_tokens?: number
+    output_tokens?: number
+    cache_creation_input_tokens?: number
+    cache_read_input_tokens?: number
+  },
+): typeof current {
+  return {
+    input_tokens: delta.input_tokens ?? current.input_tokens,
+    output_tokens: delta.output_tokens ?? current.output_tokens,
+    cache_creation_input_tokens:
+      delta.cache_creation_input_tokens !== undefined &&
+      delta.cache_creation_input_tokens > 0
+        ? delta.cache_creation_input_tokens
+        : current.cache_creation_input_tokens,
+    cache_read_input_tokens:
+      delta.cache_read_input_tokens !== undefined &&
+      delta.cache_read_input_tokens > 0
+        ? delta.cache_read_input_tokens
+        : current.cache_read_input_tokens,
+  }
+}
diff --git a/src/services/api/openai/requestBody.ts b/src/services/api/openai/requestBody.ts
index d47ccdfc80..095332cf57 100644
--- a/src/services/api/openai/requestBody.ts
+++ b/src/services/api/openai/requestBody.ts
@@ -23,7 +23,9 @@ export function isOpenAIThinkingEnabled(model: string): boolean {
   if (isEnvDefinedFalsy(process.env.OPENAI_ENABLE_THINKING)) return false
   // Explicit enable
   if (isEnvTruthy(process.env.OPENAI_ENABLE_THINKING)) return true
-  // Auto-detect from model name (DeepSeek and MiMo models support thinking mode)
+  // Auto-detect from model name (DeepSeek and MiMo models support thinking mode).
+  // Grok is intentionally excluded — Grok reasoning models reason automatically
+  // and do NOT require thinking/enable_thinking request body parameters.
   const modelLower = model.toLowerCase()
   return modelLower.includes('deepseek') || modelLower.includes('mimo')
 }