diff --git a/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.test.ts b/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.test.ts
index af95c5acf..83fc1a8e9 100644
--- a/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.test.ts
+++ b/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.test.ts
@@ -246,6 +246,32 @@ describe('AnthropicContentGenerator', () => {
       expect(headers['anthropic-beta']).toBe('experimental-x');
     });
 
+    it('honors customHeaders[anthropic-beta] under mixed-case keys (Anthropic-Beta / ANTHROPIC-BETA)', async () => {
+      // HTTP header names are case-insensitive; Anthropic SDK lower-cases
+      // headers when merging. Make sure our merge logic also matches
+      // case-insensitively so the user-configured beta flag isn't silently
+      // overwritten by the per-request value.
+      const headersUpper = await callOnce({
+        ...baseConfig,
+        reasoning: { effort: 'medium' },
+        customHeaders: { 'ANTHROPIC-BETA': 'experimental-x' },
+      });
+      expect(headersUpper['anthropic-beta']).toContain('experimental-x');
+      expect(headersUpper['anthropic-beta']).toContain(
+        'interleaved-thinking-2025-05-14',
+      );
+
+      const headersTitle = await callOnce({
+        ...baseConfig,
+        reasoning: { effort: 'medium' },
+        customHeaders: { 'Anthropic-Beta': 'experimental-y' },
+      });
+      expect(headersTitle['anthropic-beta']).toContain('experimental-y');
+      expect(headersTitle['anthropic-beta']).toContain(
+        'interleaved-thinking-2025-05-14',
+      );
+    });
+
     it('dedupes beta flags so duplicates from customHeaders are not repeated', async () => {
       const headers = await callOnce({
         ...baseConfig,
diff --git a/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.ts b/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.ts
index e08c88093..1e3b498b9 100644
--- a/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.ts
+++ b/packages/core/src/core/anthropicContentGenerator/anthropicContentGenerator.ts
@@ -208,20 +208,17 @@ export class AnthropicContentGenerator implements ContentGenerator {
    *
    * User-supplied `customHeaders['anthropic-beta']` flags are merged in (and
    * deduped) so the per-request override doesn't wipe out the existing
-   * customHeaders escape hatch for unrelated beta features.
+   * customHeaders escape hatch for unrelated beta features. The lookup is
+   * case-insensitive — HTTP header names are case-insensitive by spec, so a
+   * user-configured `Anthropic-Beta` or `ANTHROPIC-BETA` is honored too.
    */
   private buildPerRequestHeaders(
     anthropicRequest: MessageCreateParamsWithThinking,
   ): Record<string, string> | undefined {
     const betas: string[] = [];
 
-    const userBeta =
-      this.contentGeneratorConfig.customHeaders?.['anthropic-beta'];
-    if (typeof userBeta === 'string' && userBeta) {
-      for (const flag of userBeta.split(',')) {
-        const trimmed = flag.trim();
-        if (trimmed) betas.push(trimmed);
-      }
+    for (const flag of this.collectCustomBetaFlags()) {
+      betas.push(flag);
     }
 
     if (anthropicRequest.thinking) {
@@ -236,6 +233,28 @@ export class AnthropicContentGenerator implements ContentGenerator {
     return { 'anthropic-beta': unique.join(',') };
   }
 
+  /**
+   * Read every customHeaders entry whose key (case-insensitively) is
+   * `anthropic-beta` and yield the comma-separated flags from each. Multiple
+   * matching entries are concatenated; later ones may produce duplicates
+   * which the caller dedupes.
+   */
+  private collectCustomBetaFlags(): string[] {
+    const customHeaders = this.contentGeneratorConfig.customHeaders;
+    if (!customHeaders) return [];
+
+    const flags: string[] = [];
+    for (const [key, value] of Object.entries(customHeaders)) {
+      if (key.toLowerCase() !== 'anthropic-beta') continue;
+      if (typeof value !== 'string' || !value) continue;
+      for (const flag of value.split(',')) {
+        const trimmed = flag.trim();
+        if (trimmed) flags.push(trimmed);
+      }
+    }
+    return flags;
+  }
+
   private async buildRequest(
     request: GenerateContentParameters,
   ): Promise<MessageCreateParamsWithThinking> {