diff --git a/src/chat/inbound.ts b/src/chat/inbound.ts
index c72efb4715..1012354b2d 100644
--- a/src/chat/inbound.ts
+++ b/src/chat/inbound.ts
@@ -99,17 +99,8 @@ function pushSystemText(parts: string[], content: unknown): void {
   if (text) parts.push(text);
 }
 
-function toolCallsToItems(toolCalls: unknown, input: Rec[]): void {
+function toolCallsToItems(toolCalls: unknown, input: Rec[], knownNameByCallId: Map<string, string>): void {
   if (!Array.isArray(toolCalls)) return;
-  // Recover names from earlier function_call items in the same transcript when a client
-  // re-sends tool_calls with only id/arguments (replace-style merge lost function.name).
-  const knownNameByCallId = new Map<string, string>();
-  for (const item of input) {
-    if (!isRec(item) || item.type !== "function_call") continue;
-    if (typeof item.call_id === "string" && typeof item.name === "string" && item.name.length > 0) {
-      knownNameByCallId.set(item.call_id, item.name);
-    }
-  }
   for (const raw of toolCalls) {
     if (!isRec(raw)) continue;
     const fn = isRec(raw.function) ? raw.function : null;
@@ -243,6 +234,9 @@ export function chatCompletionsToResponsesBody(raw: unknown): Rec {
 
   const systemParts: string[] = [];
   const input: Rec[] = [];
+  // Recover replace-style tool calls incrementally instead of rebuilding the
+  // call-id index from the entire translated transcript for every message.
+  const knownNameByCallId = new Map<string, string>();
 
   for (const msg of raw.messages) {
     if (!isRec(msg)) continue;
@@ -260,7 +254,7 @@ export function chatCompletionsToResponsesBody(raw: unknown): Rec {
       case "assistant": {
         const blocks = assistantContentToBlocks(msg.content);
         if (blocks.length > 0) input.push({ type: "message", role: "assistant", content: blocks });
-        if (msg.tool_calls !== undefined) toolCallsToItems(msg.tool_calls, input);
+        if (msg.tool_calls !== undefined) toolCallsToItems(msg.tool_calls, input, knownNameByCallId);
         break;
       }
       case "tool": {
diff --git a/tests/chat-completions-endpoint.test.ts b/tests/chat-completions-endpoint.test.ts
index ac4112632a..24d0a56bc6 100644
--- a/tests/chat-completions-endpoint.test.ts
+++ b/tests/chat-completions-endpoint.test.ts
@@ -2141,10 +2141,6 @@ test("responsesSseToChatCompletionsSse uses finalized arguments when the item do
 });
 
 test("chatCompletionsToResponsesBody recovers tool_calls function.name from earlier call_id", () => {
-  // Simulate replace-style client history: a later assistant tool_call has id+args but empty name,
-  // while an earlier function_call in the same transcript already named it.
-  // Our translator processes messages in order; recovery looks at previously emitted function_calls.
-  // First push a prior named call via a previous assistant message, then a nameless replay.
   const body = chatCompletionsToResponsesBody({
     model: "gpt-test",
     messages: [
@@ -2158,9 +2154,10 @@ test("chatCompletionsToResponsesBody recovers tool_calls function.name from earl
       {
         role: "assistant",
         content: null,
-        // Client lost the name on a re-serialized tool_call with same id (should still recover if same turn
-        // already registered the name earlier in the same tool_calls array / prior items).
         tool_calls: [
+          // The client lost call_a's name while re-serializing an earlier message.
+          { id: "call_a", type: "function", function: { arguments: '{"cmd":"ls"}' } },
+          // Same-array recovery remains supported as well.
           { id: "call_b", type: "function", function: { name: "exec_command", arguments: '{"cmd":"pwd"}' } },
           { id: "call_b", type: "function", function: { arguments: '{"cmd":"pwd"}' } },
         ],
@@ -2168,7 +2165,54 @@ test("chatCompletionsToResponsesBody recovers tool_calls function.name from earl
     ],
   });
   const calls = (body.input as Array<Record<string, unknown>>).filter(i => i.type === "function_call");
-  expect(calls.some(c => c.call_id === "call_b" && c.name === "exec_command")).toBe(true);
+  expect(calls.filter(c => c.call_id === "call_a").map(c => c.name)).toEqual(["exec_command", "exec_command"]);
+  expect(calls.filter(c => c.call_id === "call_b").map(c => c.name)).toEqual(["exec_command", "exec_command"]);
+});
+
+test("chatCompletionsToResponsesBody indexes tool-call names once per call", () => {
+  const count = 1_000;
+  const messages: Array<Record<string, unknown>> = [{ role: "user", content: "start" }];
+  for (let i = 0; i < count; i++) {
+    messages.push({
+      role: "assistant",
+      content: null,
+      tool_calls: [{
+        id: `linear_call_${i}`,
+        type: "function",
+        function: { name: "exec_command", arguments: "{}" },
+      }],
+    });
+  }
+
+  const descriptor = Object.getOwnPropertyDescriptor(Map.prototype, "set");
+  if (!descriptor || typeof descriptor.value !== "function") throw new Error("Map.prototype.set is unavailable");
+  const nativeSet = descriptor.value as (
+    this: Map<unknown, unknown>,
+    key: unknown,
+    value: unknown,
+  ) => Map<unknown, unknown>;
+  let matchingSetCalls = 0;
+  let body: Record<string, unknown> | null = null;
+  const countingSet: typeof Map.prototype.set = function <K, V>(
+    this: Map<K, V>,
+    key: K,
+    value: V,
+  ): Map<K, V> {
+    if (typeof key === "string" && key.startsWith("linear_call_") && value === "exec_command") {
+      matchingSetCalls += 1;
+    }
+    return Reflect.apply(nativeSet, this, [key, value]) as Map<K, V>;
+  };
+
+  Object.defineProperty(Map.prototype, "set", { ...descriptor, value: countingSet });
+  try {
+    body = chatCompletionsToResponsesBody({ model: "gpt-test", messages });
+  } finally {
+    Object.defineProperty(Map.prototype, "set", descriptor);
+  }
+
+  expect((body!.input as unknown[]).length).toBe(count + 1);
+  expect(matchingSetCalls).toBe(count);
 });
 
 // Local-stack fixup regressions (Sol audit of #279, devlog 100_merge_records.md WP5):