diff --git a/src/agents/embedded-agent-runner/compact.hooks.test.ts b/src/agents/embedded-agent-runner/compact.hooks.test.ts
index 43285081e1b..29ec920e41e 100644
--- a/src/agents/embedded-agent-runner/compact.hooks.test.ts
+++ b/src/agents/embedded-agent-runner/compact.hooks.test.ts
@@ -539,11 +539,11 @@ describe("compactEmbeddedAgentSessionDirect hooks", () => {
     });
     findMockCall(
       resolveModelMock,
-      ([provider, modelId]) => provider === "openai" && modelId === "gpt-primary",
+      ([provider, modelId]) => provider === "openai-codex" && modelId === "gpt-primary",
     );
     findMockCall(
       resolveModelMock,
-      ([provider, modelId]) => provider === "openai" && modelId === "gpt-fallback",
+      ([provider, modelId]) => provider === "openai-codex" && modelId === "gpt-fallback",
     );
     expectRecordFields(mockCallArg(resolveEmbeddedAgentStreamFnMock, 1), {
       authProfileId: "openai-codex:default",
@@ -655,7 +655,7 @@ describe("compactEmbeddedAgentSessionDirect hooks", () => {
     });
 
     expect(result.ok).toBe(true);
-    expect(mockCallArg(resolveModelMock)).toBe("openai");
+    expect(mockCallArg(resolveModelMock)).toBe("openai-codex");
     expectRecordFields(mockCallArg(resolveContextWindowInfoMock), {
       provider: "openai",
       modelId: "gpt-5.5",
diff --git a/src/agents/embedded-agent-runner/compact.ts b/src/agents/embedded-agent-runner/compact.ts
index 94c4eaa8dc8..fb599b1bf2c 100644
--- a/src/agents/embedded-agent-runner/compact.ts
+++ b/src/agents/embedded-agent-runner/compact.ts
@@ -501,6 +501,7 @@ async function compactEmbeddedAgentSessionDirectOnce(
   // Keep the configured provider for harness policy, while auth/model loading below can
   // route OpenAI compaction through Codex OAuth when that runtime owns the session credentials.
   const provider = resolvedCompactionTarget.provider ?? DEFAULT_PROVIDER;
+  const runtimeProvider = resolvedCompactionTarget.runtimeProvider ?? provider;
   const modelId = resolvedCompactionTarget.model ?? DEFAULT_MODEL;
   const authProfileId = resolvedCompactionTarget.authProfileId;
   let thinkLevel: ThinkLevel = params.thinkLevel ?? "off";
@@ -541,13 +542,13 @@ async function compactEmbeddedAgentSessionDirectOnce(
     workspaceDir: resolvedWorkspace,
   });
   const { model, error, authStorage, modelRegistry } = await resolveModelAsync(
-    provider,
+    runtimeProvider,
     modelId,
     agentDir,
     params.config,
   );
   if (!model) {
-    const reason = error ?? `Unknown model: ${provider}/${modelId}`;
+    const reason = error ?? `Unknown model: ${runtimeProvider}/${modelId}`;
     return fail(reason);
   }
   let runtimeModel = model;
diff --git a/src/agents/runtime/proxy.test.ts b/src/agents/runtime/proxy.test.ts
index e2b34a50d23..8342df3c20c 100644
--- a/src/agents/runtime/proxy.test.ts
+++ b/src/agents/runtime/proxy.test.ts
@@ -75,9 +75,11 @@ describe("streamProxy", () => {
       usage,
     });
     const body = JSON.parse(String(fetchMock.mock.calls[0]?.[1]?.body)) as {
+      model?: { headers?: unknown };
       options?: { headers?: unknown };
     };
     expect(body.options).not.toHaveProperty("headers");
+    expect(body.model).not.toHaveProperty("headers");
   });
 
   it("returns an error result when EOF arrives without a terminal event", async () => {
diff --git a/src/agents/runtime/proxy.ts b/src/agents/runtime/proxy.ts
index b12c9e37df4..ff09f208cba 100644
--- a/src/agents/runtime/proxy.ts
+++ b/src/agents/runtime/proxy.ts
@@ -117,6 +117,11 @@ function buildProxyRequestOptions(options: ProxyStreamOptions): ProxySerializabl
   };
 }
 
+function sanitizeProxyModel(model: Model): Model {
+  const { headers: _headers, ...safeModel } = model;
+  return safeModel as Model;
+}
+
 export function streamProxy(
   model: Model,
   context: Context,
@@ -164,7 +169,7 @@ export function streamProxy(
           "Content-Type": "application/json",
         },
         body: JSON.stringify({
-          model,
+          model: sanitizeProxyModel(model),
           context,
           options: buildProxyRequestOptions(options),
         }),