diff --git a/CHANGELOG.md b/CHANGELOG.md
index de08c5bf11..e382e95747 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@ _In development — bullets added per PR; finalized at release._
 
 ### 🔧 Bug Fixes
 
+- **sse**: make the `anthropic-version` default-guard case-insensitive for `anthropic-compatible-*` providers, so a caller/operator-supplied `Anthropic-Version` (any casing) is no longer clobbered by a second lowercase `anthropic-version: 2023-06-01` header. (thanks @zakirkun)
+
 - **huggingface**: validate API tokens via the `whoami-v2` endpoint as a pure auth probe so fine-grained Inference-Provider tokens (valid even when model/task endpoints reject them) are no longer falsely marked invalid; only 401/403 means an invalid key, other non-OK statuses surface as transient upstream errors. (thanks @Delcado19)
 
 - **sse/kiro**: reject the Anthropic-only `[1m]` context-1m suffix in `buildKiroPayload` before it reaches AWS Bedrock — Kiro is Bedrock-backed and cannot honor the beta, so a forwarded `kr/*[1m]` model id was malformed upstream; callers now get a clear error pointing them at a direct-Anthropic provider for 1M-context routing (thanks @Delcado19).
diff --git a/open-sse/executors/default.ts b/open-sse/executors/default.ts
index 562f475ed1..b6f58a3ccc 100644
--- a/open-sse/executors/default.ts
+++ b/open-sse/executors/default.ts
@@ -491,7 +491,14 @@ export class DefaultExecutor extends BaseExecutor {
               headers["Authorization"] = `Bearer ${effectiveKey}`;
             }
           }
-          if (!headers["anthropic-version"]) {
+          // Default the anthropic-version header only when the caller/operator
+          // has not already supplied one. The lookup is case-insensitive so a
+          // pre-set "Anthropic-Version" (e.g. from this.config.headers or a
+          // custom header) is not clobbered with a duplicate lowercase entry.
+          const hasAnthropicVersion = Object.keys(headers).some(
+            (key) => key.toLowerCase() === "anthropic-version"
+          );
+          if (!hasAnthropicVersion) {
             headers["anthropic-version"] = "2023-06-01";
           }
         } else {
diff --git a/tests/unit/executor-default-base.test.ts b/tests/unit/executor-default-base.test.ts
index f4e7ed6e70..bdb6546c59 100644
--- a/tests/unit/executor-default-base.test.ts
+++ b/tests/unit/executor-default-base.test.ts
@@ -435,6 +435,43 @@ test("DefaultExecutor.buildHeaders handles GLM, default auth and anthropic-compa
   assert.equal(anthropicHeaders.Accept, "text/event-stream");
 });
 
+test("DefaultExecutor.buildHeaders keeps a caller-supplied Anthropic-Version (case-insensitive guard) for anthropic-compatible providers", () => {
+  // An operator may configure a Title-Case "Anthropic-Version" via the provider
+  // config headers. The default-guard at the anthropic-compatible-* branch must
+  // detect it case-insensitively and NOT add a second lowercase
+  // "anthropic-version" key, which undici would otherwise combine into
+  // "2025-01-01, 2023-06-01" and break the upstream request.
+  const anthropicCompat = new DefaultExecutor("anthropic-compatible-test");
+  // `config` is shared across instances via the provider registry, so snapshot
+  // and restore `config.headers` to avoid leaking the Title-Case override into
+  // other tests.
+  const originalConfigHeaders = anthropicCompat.config.headers;
+  anthropicCompat.config.headers = {
+    ...originalConfigHeaders,
+    "Anthropic-Version": "2025-01-01",
+  };
+
+  try {
+    const headers = anthropicCompat.buildHeaders({ apiKey: "anth-key" }, true);
+
+    const versionKeys = Object.keys(headers).filter(
+      (key) => key.toLowerCase() === "anthropic-version"
+    );
+    assert.equal(versionKeys.length, 1, "Duplicate anthropic-version header keys found");
+    assert.equal(headers["Anthropic-Version"], "2025-01-01");
+    assert.equal(headers["anthropic-version"], undefined);
+    assert.equal(headers["x-api-key"], "anth-key");
+  } finally {
+    anthropicCompat.config.headers = originalConfigHeaders;
+  }
+});
+
+test("DefaultExecutor.buildHeaders still defaults anthropic-version when no variant is present", () => {
+  const anthropicCompat = new DefaultExecutor("anthropic-compatible-test");
+  const headers = anthropicCompat.buildHeaders({ apiKey: "anth-key" }, true);
+  assert.equal(headers["anthropic-version"], "2023-06-01");
+});
+
 test("DefaultExecutor local OpenAI-style providers honor custom base URLs and skip empty bearer headers", () => {
   const lmStudio = new DefaultExecutor("lm-studio");
   const vllm = new DefaultExecutor("vllm");