diff --git a/src/anthropic.rs b/src/anthropic.rs
index 4ea7c89..b5b3ed6 100644
--- a/src/anthropic.rs
+++ b/src/anthropic.rs
@@ -7,9 +7,28 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
 use crate::api::{Message, Role};
+use crate::backend::Backend;
 
 const MAX_TOKENS: u32 = 80000;
 
+/// The Anthropic backend: an API key and the model to call.
+pub struct Anthropic {
+    key: String,
+    model: String,
+}
+
+impl Anthropic {
+    pub fn new(key: String, model: String) -> Self {
+        Anthropic { key, model }
+    }
+}
+
+impl Backend for Anthropic {
+    fn complete(&self, messages: &[Message]) -> anyhow::Result<String> {
+        send(&self.key, &self.model, messages)
+    }
+}
+
 /// Anthropic 400s on an empty text block, so render empty fields as a visible
 /// placeholder.
 fn field_or_placeholder(field: &str) -> &str {
@@ -77,7 +96,7 @@ enum ResponseBlock {
 }
 
 /// Send a chat-style prompt to the Claude Messages API and return the text.
-pub fn complete(api_key: &str, model: &str, messages: &[Message]) -> anyhow::Result<String> {
+fn send(api_key: &str, model: &str, messages: &[Message]) -> anyhow::Result<String> {
     let req = build_request(model, messages);
 
     tracing::debug!(
diff --git a/src/backend.rs b/src/backend.rs
new file mode 100644
index 0000000..754d0b1
--- /dev/null
+++ b/src/backend.rs
@@ -0,0 +1,64 @@
+//! The model-backend interface: one trait both providers implement, plus the
+//! single place where a `Provider` choice is turned into a ready-to-call,
+//! key-bearing backend.
+
+use anyhow::Result;
+
+use crate::anthropic::Anthropic;
+use crate::api::Message;
+use crate::config_files::{Provider, Secrets};
+use crate::openai::Openai;
+
+/// A resolved model backend — provider, key, and model already settled. Callers
+/// hand it refac's provider-agnostic [`Message`]s and get back the completion.
+///
+/// Resolved to `Box<dyn Backend>` so call sites depend only on the interface,
+/// never on which provider answered. The trait is where the upcoming tool /
+/// function-call round-trip lands (both providers support it), keeping the
+/// edit loop provider-agnostic.
+pub trait Backend {
+    /// Send the conversation and return the model's text output.
+    fn complete(&self, messages: &[Message]) -> Result<String>;
+}
+
+/// Turn a resolved provider + model into a callable backend, failing if that
+/// provider's API key is missing. This is the one spot that knows how each
+/// provider sources its key, so the rest of refac stays provider-agnostic.
+pub fn resolve(provider: Provider, model: &str, secrets: &Secrets) -> Result<Box<dyn Backend>> {
+    match provider {
+        Provider::Anthropic => {
+            let key = secrets.anthropic_api_key.clone().ok_or_else(|| {
+                anyhow::anyhow!("No Anthropic API key found. Set ANTHROPIC_API_KEY or run 'refac login'.")
+            })?;
+            Ok(Box::new(Anthropic::new(key, model.to_string())))
+        }
+        Provider::Openai => {
+            let key = secrets.openai_api_key.clone().ok_or_else(|| {
+                anyhow::anyhow!("No OpenAI API key found. Set OPENAI_API_KEY or run 'refac login'.")
+            })?;
+            Ok(Box::new(Openai::new(key, model.to_string())))
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn resolve_errors_without_a_key() {
+        let secrets = Secrets::default();
+        assert!(resolve(Provider::Anthropic, "m", &secrets).is_err());
+        assert!(resolve(Provider::Openai, "m", &secrets).is_err());
+    }
+
+    #[test]
+    fn resolve_succeeds_with_the_matching_key() {
+        let secrets = Secrets {
+            anthropic_api_key: Some("a".into()),
+            openai_api_key: Some("o".into()),
+        };
+        assert!(resolve(Provider::Anthropic, "m", &secrets).is_ok());
+        assert!(resolve(Provider::Openai, "m", &secrets).is_ok());
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 27c6f28..6b0697f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,7 @@
 mod anthropic;
 mod api;
 mod api_client;
+mod backend;
 mod config_files;
 mod openai;
 mod prompt;
@@ -109,24 +110,7 @@ fn refactor(
     let provider = config.provider(sc);
     let model = config.model(provider);
 
-    let output = match provider {
-        Provider::Anthropic => {
-            let key = sc.anthropic_api_key.as_deref().ok_or_else(|| {
-                anyhow::anyhow!(
-                    "No Anthropic API key found. Set ANTHROPIC_API_KEY or run 'refac login'."
-                )
-            })?;
-            anthropic::complete(key, &model, &messages)?
-        }
-        Provider::Openai => {
-            let key = sc.openai_api_key.as_deref().ok_or_else(|| {
-                anyhow::anyhow!(
-                    "No OpenAI API key found. Set OPENAI_API_KEY or run 'refac login'."
-                )
-            })?;
-            openai::complete(key, &model, &messages)?
-        }
-    };
+    let output = backend::resolve(provider, &model, sc)?.complete(&messages)?;
 
     log(
         LogEntry {
diff --git a/src/openai.rs b/src/openai.rs
index 1f78d00..985a21d 100644
--- a/src/openai.rs
+++ b/src/openai.rs
@@ -7,9 +7,28 @@ use serde::{Deserialize, Serialize};
 
 use crate::api::{Message, Role};
 use crate::api_client::{Client, Endpoint, Req};
+use crate::backend::Backend;
+
+/// The OpenAI backend: an API key and the model to call.
+pub struct Openai {
+    key: String,
+    model: String,
+}
+
+impl Openai {
+    pub fn new(key: String, model: String) -> Self {
+        Openai { key, model }
+    }
+}
+
+impl Backend for Openai {
+    fn complete(&self, messages: &[Message]) -> anyhow::Result<String> {
+        send(&self.key, &self.model, messages)
+    }
+}
 
 /// Send refac's messages to the OpenAI chat-completions API and return the text.
-pub fn complete(api_key: &str, model: &str, messages: &[Message]) -> anyhow::Result<String> {
+fn send(api_key: &str, model: &str, messages: &[Message]) -> anyhow::Result<String> {
     let client = Client::new(api_key);
 
     // OpenAI takes one string per message; sending each field as its own message