diff --git a/src/agents/model-compat.test.ts b/src/agents/model-compat.test.ts
index c1e79f4757a..385384ce3f7 100644
--- a/src/agents/model-compat.test.ts
+++ b/src/agents/model-compat.test.ts
@@ -338,6 +338,81 @@ describe("normalizeModelCompat", () => {
   });
 });
 
+describe("normalizeModelCompat — vLLM and SGLang providers", () => {
+  function makeVllmModel(baseUrl: string): Model<Api> {
+    return {
+      id: "Qwen3.5-27B",
+      name: "Qwen3.5-27B",
+      api: "openai-completions",
+      provider: "vllm",
+      baseUrl,
+      reasoning: false,
+      input: ["text"],
+      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+      contextWindow: 32768,
+      maxTokens: 4096,
+    } as Model<Api>;
+  }
+
+  function makeSglangModel(baseUrl: string): Model<Api> {
+    return {
+      ...makeVllmModel(baseUrl),
+      provider: "sglang",
+    } as Model<Api>;
+  }
+
+  it("defaults supportsUsageInStreaming to true for vLLM self-hosted endpoints", () => {
+    const model = makeVllmModel("http://192.168.88.35:9090/v1");
+    delete (model as { compat?: unknown }).compat;
+    const normalized = normalizeModelCompat(model);
+    expect(supportsUsageInStreaming(normalized)).toBe(true);
+  });
+
+  it("defaults supportsUsageInStreaming to true for vLLM localhost endpoints", () => {
+    const model = makeVllmModel("http://127.0.0.1:8000/v1");
+    delete (model as { compat?: unknown }).compat;
+    const normalized = normalizeModelCompat(model);
+    expect(supportsUsageInStreaming(normalized)).toBe(true);
+  });
+
+  it("defaults supportsUsageInStreaming to true for SGLang self-hosted endpoints", () => {
+    const model = makeSglangModel("http://192.168.1.10:30000/v1");
+    delete (model as { compat?: unknown }).compat;
+    const normalized = normalizeModelCompat(model);
+    expect(supportsUsageInStreaming(normalized)).toBe(true);
+  });
+
+  it("still forces supportsDeveloperRole off for vLLM endpoints", () => {
+    const model = makeVllmModel("http://192.168.88.35:9090/v1");
+    delete (model as { compat?: unknown }).compat;
+    const normalized = normalizeModelCompat(model);
+    expect(supportsDeveloperRole(normalized)).toBe(false);
+  });
+
+  it("still forces supportsStrictMode off for vLLM endpoints", () => {
+    const model = makeVllmModel("http://192.168.88.35:9090/v1");
+    delete (model as { compat?: unknown }).compat;
+    const normalized = normalizeModelCompat(model);
+    expect(supportsStrictMode(normalized)).toBe(false);
+  });
+
+  it("respects explicit supportsUsageInStreaming false override for vLLM", () => {
+    const model = makeVllmModel("http://192.168.88.35:9090/v1");
+    (model as { compat?: unknown }).compat = { supportsUsageInStreaming: false };
+    const normalized = normalizeModelCompat(model);
+    expect(supportsUsageInStreaming(normalized)).toBe(false);
+  });
+
+  it("does not mutate caller model reference for vLLM", () => {
+    const model = makeVllmModel("http://192.168.88.35:9090/v1");
+    delete (model as { compat?: unknown }).compat;
+    const normalized = normalizeModelCompat(model);
+    expect(normalized).not.toBe(model);
+    expect(supportsUsageInStreaming(model)).toBeUndefined();
+    expect(supportsUsageInStreaming(normalized)).toBe(true);
+  });
+});
+
 describe("isModernModelRef", () => {
   it("uses provider runtime hooks before fallback heuristics", () => {
     providerRuntimeMocks.resolveProviderModernModelRef.mockReturnValue(false);
diff --git a/src/agents/model-compat.ts b/src/agents/model-compat.ts
index efdad0e4958..cdd8696f703 100644
--- a/src/agents/model-compat.ts
+++ b/src/agents/model-compat.ts
@@ -82,6 +82,14 @@ function isOpenAINativeEndpoint(baseUrl: string): boolean {
   }
 }
 
+/**
+ * Self-hosted providers known to correctly support `stream_options.include_usage`
+ * (i.e., they return a terminal usage-only chunk at the end of the stream).
+ * Unlike arbitrary OpenAI-compatible proxies, these are well-tested inference
+ * engines that faithfully implement the OpenAI streaming usage spec.
+ */
+const STREAMING_USAGE_ALLOWLIST = new Set(["vllm", "sglang"]);
+
 function isAnthropicMessagesModel(model: Model<Api>): model is Model<"anthropic-messages"> {
   return model.api === "anthropic-messages";
 }
@@ -138,6 +146,13 @@ export function normalizeModelCompat(model: Model<Api>): Model<Api> {
     return model;
   }
 
+  // Self-hosted inference engines (vLLM, SGLang) correctly implement the
+  // OpenAI streaming usage spec and return a terminal usage-only chunk.
+  // Default supportsUsageInStreaming to true for these known providers so
+  // token counts are recorded even when no explicit compat override is set.
+  const providerLower = (model.provider ?? "").toLowerCase();
+  const defaultStreamingUsage = STREAMING_USAGE_ALLOWLIST.has(providerLower);
+
   // Return a new object — do not mutate the caller's model reference.
   return {
     ...model,
@@ -145,12 +160,12 @@ export function normalizeModelCompat(model: Model<Api>): Model<Api> {
       ? {
           ...compat,
           supportsDeveloperRole: forcedDeveloperRole || false,
-          ...(hasStreamingUsageOverride ? {} : { supportsUsageInStreaming: false }),
+          ...(hasStreamingUsageOverride ? {} : { supportsUsageInStreaming: defaultStreamingUsage }),
           supportsStrictMode: targetStrictMode,
         }
       : {
           supportsDeveloperRole: false,
-          supportsUsageInStreaming: false,
+          supportsUsageInStreaming: defaultStreamingUsage,
           supportsStrictMode: false,
         },
   } as typeof model;