diff --git a/open-sse/executors/base.ts b/open-sse/executors/base.ts
index 51adf9378e..330385f468 100644
--- a/open-sse/executors/base.ts
+++ b/open-sse/executors/base.ts
@@ -13,10 +13,6 @@ import { getClaudeCodeCompatibleRequestDefaults } from "@/lib/providers/requestD
 import { supportsXHighEffort } from "../config/providerModels.ts";
 import { remapToolNamesInRequest } from "../services/claudeCodeToolRemapper.ts";
 import { obfuscateInBody } from "../services/claudeCodeObfuscation.ts";
-import {
-  computeFingerprint,
-  extractFirstUserMessageText,
-} from "../services/claudeCodeFingerprint.ts";
 import { randomUUID } from "node:crypto";
 import { createHash } from "node:crypto";
 
@@ -466,9 +462,14 @@ export class BaseExecutor {
           obfuscateInBody(tb);
 
           const ccVersion = "2.1.114";
-          const messages = tb.messages as Array<{ role?: string; content?: unknown }> | undefined;
-          const msgText = extractFirstUserMessageText(messages);
-          const fp = computeFingerprint(msgText, ccVersion);
+          // Fix #1638: Use a stable fingerprint instead of message-derived one.
+          // The original computeFingerprint() hashed first-user-message chars, which
+          // changes every conversation turn. This mutated the system[] prefix on each
+          // request, invalidating Anthropic's prompt-cache prefix and forcing ~100%
+          // cache_create (vs 96% cache_read with a stable prefix). Using a per-day
+          // hash keeps the billing header format while preserving cache affinity.
+          const dayStamp = new Date().toISOString().slice(0, 10); // YYYY-MM-DD
+          const fp = createHash("sha256").update(`${dayStamp}${ccVersion}`).digest("hex").slice(0, 3);
           const billingLine = `x-anthropic-billing-header: cc_version=${ccVersion}.${fp}; cc_entrypoint=cli; cch=00000;`;
 
           if (Array.isArray(tb.system)) {
diff --git a/tests/unit/anthropic-cache-fingerprint.test.ts b/tests/unit/anthropic-cache-fingerprint.test.ts
new file mode 100644
index 0000000000..d57be63a0d
--- /dev/null
+++ b/tests/unit/anthropic-cache-fingerprint.test.ts
@@ -0,0 +1,74 @@
+/**
+ * Tests for Anthropic billing header fingerprint stability (#1638).
+ *
+ * Validates that the billing header fingerprint is stable across different
+ * messages within the same day, preventing prompt-cache prefix invalidation.
+ */
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { createHash } from "node:crypto";
+
+// Replicate the stabilized fingerprint logic from base.ts
+function computeStableFingerprint(ccVersion: string): string {
+  const dayStamp = new Date().toISOString().slice(0, 10); // YYYY-MM-DD
+  return createHash("sha256").update(`${dayStamp}${ccVersion}`).digest("hex").slice(0, 3);
+}
+
+// The old implementation for comparison
+function computeOldFingerprint(firstUserMessageText: string, version: string): string {
+  const FINGERPRINT_SALT = "59cf53e54c78";
+  const indices = [4, 7, 20];
+  const chars = indices.map((i) => firstUserMessageText[i] || "0").join("");
+  const input = `${FINGERPRINT_SALT}${chars}${version}`;
+  return createHash("sha256").update(input).digest("hex").slice(0, 3);
+}
+
+describe("Anthropic billing header fingerprint (#1638)", () => {
+  const ccVersion = "2.1.114";
+
+  it("should produce the same fingerprint for different messages (stable)", () => {
+    const fp1 = computeStableFingerprint(ccVersion);
+    const fp2 = computeStableFingerprint(ccVersion);
+    assert.equal(fp1, fp2, "Same-day fingerprints should be identical");
+  });
+
+  it("should produce a 3-character hex fingerprint", () => {
+    const fp = computeStableFingerprint(ccVersion);
+    assert.equal(fp.length, 3, "Fingerprint should be 3 chars");
+    assert.ok(/^[a-f0-9]{3}$/.test(fp), `Fingerprint '${fp}' should be lowercase hex`);
+  });
+
+  it("old implementation produces DIFFERENT fingerprints for different messages", () => {
+    const msg1 = "Hello, how can I help you with your code?";
+    const msg2 = "Please fix the bug in my application";
+    const fp1 = computeOldFingerprint(msg1, ccVersion);
+    const fp2 = computeOldFingerprint(msg2, ccVersion);
+    assert.notEqual(fp1, fp2, "Old method should differ per message — this was the bug");
+  });
+
+  it("new implementation produces SAME fingerprint regardless of message content", () => {
+    // The new implementation doesn't use message content at all
+    const fp1 = computeStableFingerprint(ccVersion);
+    const fp2 = computeStableFingerprint(ccVersion);
+    const fp3 = computeStableFingerprint(ccVersion);
+    assert.equal(fp1, fp2);
+    assert.equal(fp2, fp3);
+  });
+
+  it("billing header line should be deterministic within the same day", () => {
+    const fp = computeStableFingerprint(ccVersion);
+    const billingLine1 = `x-anthropic-billing-header: cc_version=${ccVersion}.${fp}; cc_entrypoint=cli; cch=00000;`;
+    const billingLine2 = `x-anthropic-billing-header: cc_version=${ccVersion}.${fp}; cc_entrypoint=cli; cch=00000;`;
+    assert.equal(billingLine1, billingLine2, "Billing lines should be byte-identical");
+  });
+
+  it("should produce different fingerprints for different days", () => {
+    // Simulate different days by computing manually
+    const day1 = "2026-04-27";
+    const day2 = "2026-04-28";
+    const fp1 = createHash("sha256").update(`${day1}${ccVersion}`).digest("hex").slice(0, 3);
+    const fp2 = createHash("sha256").update(`${day2}${ccVersion}`).digest("hex").slice(0, 3);
+    // They should be different (extremely high probability with SHA-256)
+    assert.notEqual(fp1, fp2, "Different days should produce different fingerprints");
+  });
+});