diff --git a/CHANGELOG.md b/CHANGELOG.md
index 92584f6b..dd7dc91a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,11 @@ All notable changes to this project are documented here. The format follows
 
 ### Fixed
 
+- **Codex `-fast` models request priority service tier.** Incoming ids like `gpt-5.4-mini-fast`
+  already stripped to the base model, but the Responses body never set `service_tier`. Codex
+  upstream requests now send `"service_tier": "priority"` when the client asked for `-fast`
+  (Grok model ids that literally contain `-fast` are unchanged).
+
 - **Codex hosted web search results reach Claude Code.** After the request-side `allowed_tools`
   fix, Codex `web_search_call` items were still dropped on the way back, so the client never saw
   `server_tool_use` / `web_search_tool_result` blocks or `usage.server_tool_use`. The reducer now
diff --git a/crates/llmtrim-cli/src/reroute/codex.rs b/crates/llmtrim-cli/src/reroute/codex.rs
index b61b90df..c2cba977 100644
--- a/crates/llmtrim-cli/src/reroute/codex.rs
+++ b/crates/llmtrim-cli/src/reroute/codex.rs
@@ -35,11 +35,22 @@ const WEB_SEARCH_TOOL: &str = "web_search_20250305";
 ///
 /// `model` is already resolved to the upstream id (the caller stripped `[1m]`/`-fast` and mapped
 /// tiers). `session_id` is the `x-claude-code-session-id` header value if present; it becomes the
-/// Responses `prompt_cache_key`.
+/// Responses `prompt_cache_key`. Convenience wrapper: no priority service tier.
 pub fn build_request_body(
     anthropic: &Value,
     model: &str,
     session_id: Option<&str>,
+) -> Result<Value> {
+    build_request_body_with_priority(anthropic, model, session_id, false)
+}
+
+/// Like [`build_request_body`], but when `priority` is true sets `service_tier: "priority"`
+/// (Codex mapping for an incoming model id ending in `-fast`).
+pub(crate) fn build_request_body_with_priority(
+    anthropic: &Value,
+    model: &str,
+    session_id: Option<&str>,
+    priority: bool,
 ) -> Result<Value> {
     let mut body = Map::new();
     body.insert("model".into(), json!(model));
@@ -78,6 +89,11 @@ pub fn build_request_body(
         body.insert("prompt_cache_key".into(), json!(sid));
     }
 
+    // Incoming `-fast` (stripped by the caller) maps to Codex priority service tier, not a model id.
+    if priority {
+        body.insert("service_tier".into(), json!("priority"));
+    }
+
     // text = { verbosity, format? }
     let mut text = Map::new();
     text.insert("verbosity".into(), json!("low"));
@@ -1747,6 +1763,22 @@ mod tests {
         assert!(body.get("include").is_none());
     }
 
+    #[test]
+    fn priority_flag_sets_service_tier() {
+        let body = build_request_body_with_priority(
+            &json!({ "messages": [] }),
+            "gpt-5.4-mini",
+            None,
+            true,
+        )
+        .expect("build");
+        assert_eq!(body["model"], "gpt-5.4-mini");
+        assert_eq!(body["service_tier"], "priority");
+        let plain =
+            build_request_body(&json!({ "messages": [] }), "gpt-5.4-mini", None).expect("build");
+        assert!(plain.get("service_tier").is_none());
+    }
+
     #[test]
     fn max_tokens_and_sampling_dropped_static_fields_present() {
         let body = build_request_body(
diff --git a/crates/llmtrim-cli/src/reroute/mod.rs b/crates/llmtrim-cli/src/reroute/mod.rs
index 735fb41b..e42e301a 100644
--- a/crates/llmtrim-cli/src/reroute/mod.rs
+++ b/crates/llmtrim-cli/src/reroute/mod.rs
@@ -79,7 +79,15 @@ pub(crate) fn build_upstream_for_model(
     }
     let (host, path, body, headers) = match provider {
         SubProvider::Codex => {
-            let b = codex::build_request_body(anthropic_body, &model, session_id)?;
+            // `-fast` is a service-tier hint for Codex (not a model id); Grok ids keep `-fast`
+            // literally and never take this path.
+            let priority = incoming_wants_priority_tier(incoming);
+            let b = codex::build_request_body_with_priority(
+                anthropic_body,
+                &model,
+                session_id,
+                priority,
+            )?;
             let h = codex::request_headers_with_mode(
                 &token.access,
                 token.account_id.as_deref(),
@@ -448,6 +456,13 @@ pub fn normalize_incoming(model: &str) -> (String, bool) {
     }
 }
 
+/// Whether the incoming Anthropic model id requested Codex priority service tier via a trailing
+/// `-fast` (after stripping `[1m]`). Callers must only apply this on the Codex path — Grok model
+/// ids can literally contain `-fast` (e.g. `grok-composer-2.5-fast`).
+pub fn incoming_wants_priority_tier(incoming: &str) -> bool {
+    normalize_incoming(incoming).1
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -635,4 +650,66 @@ mod tests {
             serde_json::json!([{ "type": "web_search" }])
         );
     }
+
+    #[test]
+    fn codex_fast_suffix_sets_priority_service_tier() {
+        let token = auth::TokenSet {
+            access: "tok".into(),
+            account_id: None,
+        };
+        let body = serde_json::json!({
+            "model": "gpt-5.4-mini-fast",
+            "messages": []
+        });
+        let up = build_upstream(SubProvider::Codex, &body, &BTreeMap::new(), &token, None)
+            .expect("build");
+        assert_eq!(up.model, "gpt-5.4-mini");
+        let parsed: serde_json::Value = serde_json::from_slice(&up.body).expect("json");
+        assert_eq!(parsed["model"], "gpt-5.4-mini");
+        assert_eq!(parsed["service_tier"], "priority");
+    }
+
+    #[test]
+    fn codex_non_fast_omits_service_tier() {
+        let token = auth::TokenSet {
+            access: "tok".into(),
+            account_id: None,
+        };
+        let body = serde_json::json!({
+            "model": "gpt-5.4-mini",
+            "messages": []
+        });
+        let up = build_upstream(SubProvider::Codex, &body, &BTreeMap::new(), &token, None)
+            .expect("build");
+        let parsed: serde_json::Value = serde_json::from_slice(&up.body).expect("json");
+        assert_eq!(parsed["model"], "gpt-5.4-mini");
+        assert!(parsed.get("service_tier").is_none());
+    }
+
+    #[test]
+    fn grok_fast_model_id_keeps_literal_suffix_without_service_tier() {
+        let token = auth::TokenSet {
+            access: "tok".into(),
+            account_id: None,
+        };
+        let body = serde_json::json!({
+            "model": "grok-composer-2.5-fast",
+            "messages": []
+        });
+        let up = build_upstream(SubProvider::Grok, &body, &BTreeMap::new(), &token, None)
+            .expect("build");
+        assert_eq!(up.model, "grok-composer-2.5-fast");
+        let parsed: serde_json::Value = serde_json::from_slice(&up.body).expect("json");
+        assert_eq!(parsed["model"], "grok-composer-2.5-fast");
+        assert!(parsed.get("service_tier").is_none());
+    }
+
+    #[test]
+    fn incoming_wants_priority_tier_detects_fast_suffix() {
+        assert!(incoming_wants_priority_tier("gpt-5.4-mini-fast"));
+        assert!(incoming_wants_priority_tier("gpt-5.5-fast[1m]"));
+        assert!(!incoming_wants_priority_tier("gpt-5.4-mini"));
+        // Grok ids also end in -fast; the helper reports true, but only the Codex path applies it.
+        assert!(incoming_wants_priority_tier("grok-composer-2.5-fast"));
+    }
 }