diff --git a/containers/api-proxy/providers/copilot.js b/containers/api-proxy/providers/copilot.js
index a7bc2998..30860822 100644
--- a/containers/api-proxy/providers/copilot.js
+++ b/containers/api-proxy/providers/copilot.js
@@ -21,6 +21,7 @@ const {
   createAdapterMethods,
   composeBodyTransforms,
 } = require('../proxy-utils');
+const { sanitizeNullToolCallTypes } = require('../body-transform');
 const { URL } = require('url');
 
 // AWF injects this sentinel value into the agent environment for credential isolation.
@@ -173,51 +174,6 @@ function deriveGitHubApiBasePath(env = process.env) {
   }
 }
 
-/**
- * Normalize OpenAI-style tool calls that omit the required type field.
- *
- * Some Copilot/OpenAI-compatible responses can echo tool_calls entries with a
- * null/undefined `type`, which later causes upstream validation failures when
- * the same message history is sent back. This transform patches only
- * function-style tool calls by setting `type: "function"`.
- *
- * @param {Buffer} body - Raw request body
- * @returns {Buffer|null} Updated body or null when no changes are needed
- */
-function normalizeNullTypeToolCalls(body) {
-  let parsed;
-  try {
-    parsed = JSON.parse(body.toString('utf8'));
-  } catch {
-    return null;
-  }
-
-  if (!parsed || typeof parsed !== 'object' || Array.isArray(parsed) || !Array.isArray(parsed.messages)) {
-    return null;
-  }
-
-  let changed = false;
-  for (const message of parsed.messages) {
-    if (!message || typeof message !== 'object' || Array.isArray(message) || !Array.isArray(message.tool_calls)) {
-      continue;
-    }
-
-    for (const toolCall of message.tool_calls) {
-      if (!toolCall || typeof toolCall !== 'object' || Array.isArray(toolCall)) continue;
-      const hasFunctionPayload =
-        toolCall.function &&
-        typeof toolCall.function === 'object' &&
-        !Array.isArray(toolCall.function);
-      if (toolCall.type == null && hasFunctionPayload) {
-        toolCall.type = 'function';
-        changed = true;
-      }
-    }
-  }
-
-  return changed ? Buffer.from(JSON.stringify(parsed)) : null;
-}
-
 /**
  * Create the GitHub Copilot provider adapter.
  *
@@ -236,7 +192,7 @@ function createCopilotAdapter(env, deps = {}) {
 
   const bodyTransform = composeBodyTransforms(
     deps.bodyTransform || null,
-    normalizeNullTypeToolCalls
+    (body) => { const result = sanitizeNullToolCallTypes(body); return result ? result.body : null; }
   );
 
   // Pre-computed models path used by getModelsFetchConfig and getReflectionInfo.
@@ -410,7 +366,6 @@ module.exports = {
     deriveCopilotApiTarget,
     deriveGitHubApiTarget,
     deriveGitHubApiBasePath,
-    normalizeNullTypeToolCalls,
     COPILOT_PLACEHOLDER_TOKEN,
   },
 };
diff --git a/containers/api-proxy/server.auth.test.js b/containers/api-proxy/server.auth.test.js
index f2f59787..4a51ae1a 100644
--- a/containers/api-proxy/server.auth.test.js
+++ b/containers/api-proxy/server.auth.test.js
@@ -6,9 +6,10 @@
 
 const { shouldStripHeader } = require('./proxy-utils');
 const {
-  _testing: { resolveCopilotAuthToken, resolveApiKey, stripBearerPrefix, normalizeNullTypeToolCalls, COPILOT_PLACEHOLDER_TOKEN },
+  _testing: { resolveCopilotAuthToken, resolveApiKey, stripBearerPrefix, COPILOT_PLACEHOLDER_TOKEN },
   createCopilotAdapter,
 } = require('./providers/copilot');
+const { sanitizeNullToolCallTypes } = require('./body-transform');
 
 describe('shouldStripHeader', () => {
   it('should strip authorization header', () => {
@@ -170,7 +171,7 @@ describe('resolveApiKey', () => {
   });
 });
 
-describe('normalizeNullTypeToolCalls', () => {
+describe('sanitizeNullToolCallTypes (via copilot body transform)', () => {
   it('normalizes null tool_call type to "function" in outgoing message history', () => {
     const input = Buffer.from(JSON.stringify({
       model: 'gpt-5.4',
@@ -188,9 +189,9 @@ describe('normalizeNullTypeToolCalls', () => {
       ],
     }));
 
-    const transformed = normalizeNullTypeToolCalls(input);
-    expect(transformed).not.toBeNull();
-    const parsed = JSON.parse(transformed.toString('utf8'));
+    const result = sanitizeNullToolCallTypes(input);
+    expect(result).not.toBeNull();
+    const parsed = JSON.parse(result.body.toString('utf8'));
     expect(parsed.messages[0].tool_calls[0].type).toBe('function');
   });
 
@@ -210,7 +211,7 @@ describe('normalizeNullTypeToolCalls', () => {
       ],
     }));
 
-    expect(normalizeNullTypeToolCalls(input)).toBeNull();
+    expect(sanitizeNullToolCallTypes(input)).toBeNull();
   });
 });