diff --git a/.changeset/fuzzy-dingos-key.md b/.changeset/fuzzy-dingos-key.md
new file mode 100644
index 0000000000..2eec4a85e8
--- /dev/null
+++ b/.changeset/fuzzy-dingos-key.md
@@ -0,0 +1,5 @@
+---
+"eve": patch
+---
+
+Prevent connection token cache collisions when a principal issuer or ID contains `:` or `%`.
diff --git a/packages/eve/src/public/channels/eve-forwarded-principal.integration.test.ts b/packages/eve/src/public/channels/eve-forwarded-principal.integration.test.ts
index 5f144772d8..d2e66a7fb2 100644
--- a/packages/eve/src/public/channels/eve-forwarded-principal.integration.test.ts
+++ b/packages/eve/src/public/channels/eve-forwarded-principal.integration.test.ts
@@ -158,7 +158,7 @@ describe("eveChannel forwarded principal → runtime principal", () => {
       type: "user",
     });
     // The audit attribute never enters Connect token-cache keying.
-    expect(principalKey(principal)).toBe("user:slack:slack:U123");
+    expect(principalKey(principal)).toBe("user:slack:slack%3AU123");
   });
 
   it("resolves the transport service principal (and fails Connect) without forwarding", async () => {
diff --git a/packages/eve/src/runtime/connections/principal.test.ts b/packages/eve/src/runtime/connections/principal.test.ts
index 08fd9957ac..99869cef3f 100644
--- a/packages/eve/src/runtime/connections/principal.test.ts
+++ b/packages/eve/src/runtime/connections/principal.test.ts
@@ -59,6 +59,36 @@ describe("principalKey", () => {
   it("keys an issuerless native Vercel user by its user id", () => {
     expect(principalKey({ id: "user_123", type: "user" })).toBe("user:user_123");
   });
+
+  it("does not alias an issuer with an issuerless id containing the separator", () => {
+    const issued = principalKey({ id: "U123", issuer: "slack", type: "user" });
+    const issuerless = principalKey({ id: "slack:U123", type: "user" });
+
+    expect(issued).not.toBe(issuerless);
+    expect(issuerless).toBe("user:slack%3AU123");
+  });
+
+  it("does not alias a separator in the issuer with one in the id", () => {
+    const separatorInIssuer = principalKey({
+      id: "U123",
+      issuer: "slack:webhook",
+      type: "user",
+    });
+    const separatorInId = principalKey({ id: "webhook:U123", issuer: "slack", type: "user" });
+
+    expect(separatorInIssuer).not.toBe(separatorInId);
+    expect(separatorInIssuer).toBe("user:slack%3Awebhook:U123");
+    expect(separatorInId).toBe("user:slack:webhook%3AU123");
+  });
+
+  it("does not alias a literal percent-encoded separator with a separator", () => {
+    const encoded = principalKey({ id: "a%3Ab", type: "user" });
+    const separator = principalKey({ id: "a:b", type: "user" });
+
+    expect(encoded).not.toBe(separator);
+    expect(encoded).toBe("user:a%253Ab");
+    expect(separator).toBe("user:a%3Ab");
+  });
 });
 
 describe("resolveConnectionPrincipal", () => {
diff --git a/packages/eve/src/runtime/connections/principal.ts b/packages/eve/src/runtime/connections/principal.ts
index a504ce608c..9eb073081a 100644
--- a/packages/eve/src/runtime/connections/principal.ts
+++ b/packages/eve/src/runtime/connections/principal.ts
@@ -27,15 +27,23 @@ import { isVercelOidcIssuer } from "#shared/vercel-project.js";
  *   would otherwise alias to the same cache slot.
  * - `{ type: "user", id }` → `"user:${id}"`. This is the native
  *   Vercel Connect user projection.
+ *
+ * `%` and `:` are percent-encoded within each user segment so the
+ * separator cannot make distinct principals render to the same key.
  */
 export function principalKey(principal: ConnectionPrincipal): string {
   if (principal.type === "app") {
     return "app";
   }
+  const id = encodePrincipalKeySegment(principal.id);
   if (principal.issuer === undefined) {
-    return `user:${principal.id}`;
+    return `user:${id}`;
   }
-  return `user:${principal.issuer}:${principal.id}`;
+  return `user:${encodePrincipalKeySegment(principal.issuer)}:${id}`;
+}
+
+function encodePrincipalKeySegment(value: string): string {
+  return value.replaceAll("%", "%25").replaceAll(":", "%3A");
 }
 
 /**