From d51850b31f6e5f34f5d15d460fc15bcd91b4917a Mon Sep 17 00:00:00 2001
From: lawrencecchen <54008264+lawrencecchen@users.noreply.github.com>
Date: Tue, 3 Nov 2026 11:31:24 +0000
Subject: [PATCH] fix(acpmux): opaque agent id takes its hex prefix with get,
 not a string slice (panic-ratchet red since #33209)

#33209 (85ba3b53299) added `&hex[..HEX_LEN]` in opaque_agent_id.rs. The panic
ratchet denies clippy::string_slice, and acpmux went from 94 to 95 hits, so the
feat tip panic-ratchet is red since 92bf57ad70c9. The hex is always 64 ASCII
digits; `hex.get(..HEX_LEN)` returns the same 20 digits with no panic path.
The module's existing tests check the id length and value.
---
 cmux-tui/crates/acpmux/src/opaque_agent_id.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/cmux-tui/crates/acpmux/src/opaque_agent_id.rs b/cmux-tui/crates/acpmux/src/opaque_agent_id.rs
index 9334d00f3e7b..ccdc1967a056 100644
--- a/cmux-tui/crates/acpmux/src/opaque_agent_id.rs
+++ b/cmux-tui/crates/acpmux/src/opaque_agent_id.rs
@@ -79,7 +79,8 @@ impl Key {
         let msg = format!("{}\0{harness}\0{session}", host.to_ascii_lowercase());
         let mac = cmux_local_auth::frontend_proof::hmac_sha256(&self.0, msg.as_bytes());
         let hex = cmux_local_auth::frontend_proof::hex(&mac);
-        format!("{PREFIX}{}", &hex[..HEX_LEN])
+        // `hex` is 64 ASCII digits, so the prefix always exists; `get` keeps the panic ratchet at zero.
+        format!("{PREFIX}{}", hex.get(..HEX_LEN).unwrap_or(&hex))
     }
 }