diff --git a/crates/pidgin-napi/schema/api.json b/crates/pidgin-napi/schema/api.json
index eef6b28..f534d1a 100644
--- a/crates/pidgin-napi/schema/api.json
+++ b/crates/pidgin-napi/schema/api.json
@@ -938,6 +938,27 @@
               "name": "fuzzyFilter"
             }
           }
+        },
+        {
+          "name": "detectSupportedImageMimeType",
+          "doc": "`detectSupportedImageMimeType` (utils/mime.ts): sniff a supported image MIME\ntype from magic bytes, or `null`.",
+          "shape": "unary",
+          "infallible": true,
+          "readonly": true,
+          "params": [
+            {
+              "name": "buffer",
+              "type": "bytes"
+            }
+          ],
+          "returns": {
+            "nullable": "string"
+          },
+          "bindings": {
+            "node": {
+              "name": "detectSupportedImageMimeType"
+            }
+          }
         }
       ]
     },
diff --git a/crates/pidgin-napi/src/core_impl.rs b/crates/pidgin-napi/src/core_impl.rs
index b64d698..5cdb389 100644
--- a/crates/pidgin-napi/src/core_impl.rs
+++ b/crates/pidgin-napi/src/core_impl.rs
@@ -39,6 +39,11 @@
 ///   (JS `number`); `fuzzyFilter` returns the surviving indices as `uint32` (JS
 ///   `number`), widened from the engine's `usize` at the seam — the JS-visible
 ///   scores and indices are identical to the pre-swap wrappers.
+/// - the coding-agent mime op (`detectSupportedImageMimeType`) routes into
+///   `pidgin_coding::utils::mime`, backing the native `utils/mime.ts` shim. The
+///   image byte prefix crosses as the `bytes` scalar — spelled `Uint8Array` on
+///   the JS side (a read-only view) — and the sniffed MIME type crosses back as
+///   `string | null`, identical to the pre-swap hand-written wrapper.
 pub struct PidginImpl;
 
 impl crate::generated::PidginCore for PidginImpl {
@@ -284,6 +289,12 @@ impl crate::generated::PidginCore for PidginImpl {
             .map(|i| i as u32)
             .collect()
     }
+
+    fn detect_supported_image_mime_type(
+        buffer: napi::bindgen_prelude::Uint8Array,
+    ) -> Option<String> {
+        pidgin_coding::utils::mime::detect_supported_image_mime_type(&buffer).map(|s| s.to_string())
+    }
 }
 
 // --- coding-agent session-cwd seam (core/session-cwd.ts) --------------------
diff --git a/crates/pidgin-napi/src/generated.rs b/crates/pidgin-napi/src/generated.rs
index c1da374..0f4aa7f 100644
--- a/crates/pidgin-napi/src/generated.rs
+++ b/crates/pidgin-napi/src/generated.rs
@@ -12,6 +12,9 @@ fn err(e: impl std::fmt::Display) -> napi::Error {
     napi::Error::from_reason(e.to_string())
 }
 
+/// Bulk bytes cross into JS as a Buffer (Arrow IPC payloads and friends).
+pub type Bytes = napi::bindgen_prelude::Buffer;
+
 /// Result of [`slice_with_width`]; serialized to `{ text, width }`.
 #[napi(object)]
 #[derive(Clone)]
@@ -110,6 +113,9 @@ pub trait PidginCore: Sized + Send + Sync + 'static {
     fn has_trust_requiring_project_resources(cwd: String, home_dir: String) -> bool;
     fn fuzzy_match(query: String, text: String) -> FuzzyMatchResult;
     fn fuzzy_filter(texts: Vec<String>, query: String) -> Vec<u32>;
+    fn detect_supported_image_mime_type(
+        buffer: napi::bindgen_prelude::Uint8Array,
+    ) -> Option<String>;
 }
 
 /// The `KeybindingsManagerCore` contract — implement over the engine in `crate::core_impl`.
@@ -450,6 +456,15 @@ pub fn fuzzy_filter(texts: Vec<String>, query: String) -> Vec<u32> {
     <crate::core_impl::PidginImpl as PidginCore>::fuzzy_filter(texts, query)
 }
 
+/// `detectSupportedImageMimeType` (utils/mime.ts): sniff a supported image MIME
+/// type from magic bytes, or `null`.
+#[napi(js_name = "detectSupportedImageMimeType")]
+pub fn detect_supported_image_mime_type(
+    buffer: napi::bindgen_prelude::Uint8Array,
+) -> Option<String> {
+    <crate::core_impl::PidginImpl as PidginCore>::detect_supported_image_mime_type(buffer)
+}
+
 /// The Rust-backed keybindings core, exposed to JavaScript as
 /// `KeybindingsManagerCore`.
 #[napi]
diff --git a/crates/pidgin-napi/src/lib.rs b/crates/pidgin-napi/src/lib.rs
index 9d976aa..a4c3c2b 100644
--- a/crates/pidgin-napi/src/lib.rs
+++ b/crates/pidgin-napi/src/lib.rs
@@ -185,19 +185,12 @@ pub fn anthropic_parse_sse_stream(
 
 // --- coding-agent utils layer -----------------------------------------------
 //
-// Thin wrappers over `pidgin_coding::utils::*`, backing the hand-written native
-// shims under conformance/shims/packages/coding-agent/src/utils/. Each mirrors
-// the pi export it replaces; the shims re-export the un-ported surface from the
-// preserved pi original and override only these symbols.
-
-/// `detectSupportedImageMimeType` (utils/mime.ts): sniff a supported image MIME
-/// type from magic bytes, or `null`.
-#[napi(js_name = "detectSupportedImageMimeType")]
-pub fn detect_supported_image_mime_type(
-    buffer: napi::bindgen_prelude::Uint8Array,
-) -> Option<String> {
-    pidgin_coding::utils::mime::detect_supported_image_mime_type(&buffer).map(|s| s.to_string())
-}
+// The `detectSupportedImageMimeType` byte-sniffer (utils/mime.ts) now generates
+// from the fluessig api schema through `crate::generated` + `crate::core_impl`,
+// routing into `pidgin_coding::utils::mime`. Its image buffer arg is authored as
+// the fluessig `bytes` scalar (spelled `Uint8Array` in the node `.d.ts`). The
+// hand-written `#[napi]` export that lived here was deleted; edit `schema/api.json`
+// and rerun `regen.sh` instead of re-adding it.
 
 // --- coding-agent http-dispatcher layer -------------------------------------
 //