diff --git a/src/adapters/anthropic.ts b/src/adapters/anthropic.ts
index 1ecf9e1c35b..420c0206755 100644
--- a/src/adapters/anthropic.ts
+++ b/src/adapters/anthropic.ts
@@ -63,6 +63,14 @@ function usageFromAnthropic(usage: Record<string, number> | undefined): OcxUsage
   };
 }
 
+function mergeAnthropicUsage(
+  base: Record<string, number> | undefined,
+  next: Record<string, number> | undefined,
+): Record<string, number> | undefined {
+  if (!next) return base;
+  return { ...(base ?? {}), ...next };
+}
+
 function buildToolNameTransforms(provider: OcxProviderConfig): { toWire: (name: string) => string; fromWire: (name: string) => string } {
   if (provider.authMode === "oauth") {
     return { toWire: applyClaudeToolPrefix, fromWire: stripClaudeToolPrefix };
@@ -269,6 +277,14 @@ export function createAnthropicAdapter(provider: OcxProviderConfig): ProviderAda
       let currentBlockType = "";
       let currentToolCallId = "";
       let currentToolCallName = "";
+      let pendingUsage: Record<string, number> | undefined;
+      let emittedDone = false;
+
+      const emitDone = function* (): Generator<AdapterEvent> {
+        if (emittedDone) return;
+        emittedDone = true;
+        yield { type: "done", usage: usageFromAnthropic(pendingUsage) };
+      };
 
       try {
         while (true) {
@@ -298,6 +314,11 @@ export function createAnthropicAdapter(provider: OcxProviderConfig): ProviderAda
             }
 
             switch (currentEventType || data.type) {
+              case "message_start": {
+                const message = data.message as { usage?: Record<string, number> } | undefined;
+                pendingUsage = mergeAnthropicUsage(pendingUsage, message?.usage);
+                break;
+              }
               case "content_block_start": {
                 const block = data.content_block as { type: string; id?: string; name?: string } | undefined;
                 if (!block) break;
@@ -331,15 +352,11 @@ export function createAnthropicAdapter(provider: OcxProviderConfig): ProviderAda
               }
               case "message_delta": {
                 const usage = data.usage as Record<string, number> | undefined;
-                if (usage) {
-                  yield {
-                    type: "done",
-                    usage: usageFromAnthropic(usage),
-                  };
-                }
+                pendingUsage = mergeAnthropicUsage(pendingUsage, usage);
                 break;
               }
               case "message_stop": {
+                yield* emitDone();
                 break;
               }
               case "error": {
@@ -351,6 +368,7 @@ export function createAnthropicAdapter(provider: OcxProviderConfig): ProviderAda
             currentEventType = "";
           }
         }
+        if (pendingUsage && !emittedDone) yield* emitDone();
       } finally {
         reader.releaseLock();
       }
diff --git a/tests/adapter-usage.test.ts b/tests/adapter-usage.test.ts
index d27d1bc00cc..07c7ea10aeb 100644
--- a/tests/adapter-usage.test.ts
+++ b/tests/adapter-usage.test.ts
@@ -118,6 +118,44 @@ describe("usage and content retention (F2)", () => {
     expect(events.at(-1)).toEqual({ type: "done", usage: { inputTokens: 5, outputTokens: 1 } });
   });
 
+  test("anthropic stream merges message_start input usage with message_delta output usage", async () => {
+    const adapter = createAnthropicAdapter({ ...provider, adapter: "anthropic" });
+    const response = new Response([
+      'event: message_start\n',
+      'data: {"type":"message_start","message":{"usage":{"input_tokens":20,"cache_read_input_tokens":3,"cache_creation_input_tokens":2}}}\n\n',
+      'event: content_block_delta\n',
+      'data: {"type":"content_block_delta","delta":{"type":"text_delta","text":"hi"}}\n\n',
+      'event: message_delta\n',
+      'data: {"type":"message_delta","usage":{"output_tokens":4}}\n\n',
+      'event: message_stop\n',
+      'data: {"type":"message_stop"}\n\n',
+    ].join(""));
+
+    const events = [];
+    for await (const event of adapter.parseStream(response)) events.push(event);
+    const dones = events.filter(e => e.type === "done");
+    expect(events).toContainEqual({ type: "text_delta", text: "hi" });
+    expect(dones).toHaveLength(1);
+    expect(dones[0]).toEqual({
+      type: "done",
+      usage: { inputTokens: 20, outputTokens: 4, cachedInputTokens: 5 },
+    });
+  });
+
+  test("anthropic stream emits terminal usage on EOF when message_stop is missing", async () => {
+    const adapter = createAnthropicAdapter({ ...provider, adapter: "anthropic" });
+    const response = new Response([
+      'event: message_start\n',
+      'data: {"type":"message_start","message":{"usage":{"input_tokens":7}}}\n\n',
+      'event: message_delta\n',
+      'data: {"type":"message_delta","usage":{"output_tokens":1}}\n\n',
+    ].join(""));
+
+    const events = [];
+    for await (const event of adapter.parseStream(response)) events.push(event);
+    expect(events.at(-1)).toEqual({ type: "done", usage: { inputTokens: 7, outputTokens: 1 } });
+  });
+
   test("google emits exactly one done carrying usage", async () => {
     const adapter = createGoogleAdapter({ ...provider, adapter: "google" });
     const response = new Response(