diff --git a/Cargo.lock b/Cargo.lock
index 7183238..cc0d8cf 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -26,6 +26,16 @@ version = "1.0.101"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5f0e0fee31ef5ed1ba1316088939cea399010ed7731dba877ed44aeb407a75ea"
 
+[[package]]
+name = "assert-json-diff"
+version = "2.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47e4f2b81832e72834d7518d8487a0396a28cc408186a2e8854c0f98011faf12"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "async-stream"
 version = "0.3.6"
@@ -207,6 +217,15 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "colored"
+version = "3.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "faf9468729b8cbcea668e36183cb69d317348c2e08e994829fb56ebfdfbaac34"
+dependencies = [
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "combine"
 version = "4.6.7"
@@ -1212,6 +1231,31 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "mockito"
+version = "1.7.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "90820618712cab19cfc46b274c6c22546a82affcb3c3bdf0f29e3db8e1bb92c0"
+dependencies = [
+ "assert-json-diff",
+ "bytes 1.11.1",
+ "colored",
+ "futures-core",
+ "http",
+ "http-body",
+ "http-body-util",
+ "hyper",
+ "hyper-util",
+ "log",
+ "pin-project-lite",
+ "rand",
+ "regex",
+ "serde_json",
+ "serde_urlencoded",
+ "similar",
+ "tokio",
+]
+
 [[package]]
 name = "multer"
 version = "3.1.0"
@@ -2085,6 +2129,12 @@ version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0fda2ff0d084019ba4d7c6f371c95d8fd75ce3524c3cb8fb653a3023f6323e64"
 
+[[package]]
+name = "similar"
+version = "2.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbbb5d9659141646ae647b42fe094daf6c6192d1620870b449d9557f748b2daa"
+
 [[package]]
 name = "slab"
 version = "0.4.12"
@@ -2144,6 +2194,7 @@ dependencies = [
  "homedir",
  "ignore",
  "mime_guess",
+ "mockito",
  "notify",
  "poem",
  "poem-openapi",
@@ -2333,6 +2384,7 @@ dependencies = [
  "bytes 1.11.1",
  "libc",
  "mio",
+ "parking_lot",
  "pin-project-lite",
  "socket2",
  "tokio-macros",
diff --git a/server/Cargo.toml b/server/Cargo.toml
index 16a1ea9..37e9fb0 100644
--- a/server/Cargo.toml
+++ b/server/Cargo.toml
@@ -32,3 +32,4 @@ walkdir = { workspace = true }
 [dev-dependencies]
 tempfile = { workspace = true }
 tokio-tungstenite = { workspace = true }
+mockito = "1"
diff --git a/server/src/llm/providers/anthropic.rs b/server/src/llm/providers/anthropic.rs
index abd9ee1..8179d49 100644
--- a/server/src/llm/providers/anthropic.rs
+++ b/server/src/llm/providers/anthropic.rs
@@ -13,6 +13,7 @@ const ANTHROPIC_VERSION: &str = "2023-06-01";
 pub struct AnthropicProvider {
     api_key: String,
     client: reqwest::Client,
+    api_url: String,
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -66,6 +67,16 @@ impl AnthropicProvider {
         Self {
             api_key,
             client: reqwest::Client::new(),
+            api_url: ANTHROPIC_API_URL.to_string(),
+        }
+    }
+
+    #[cfg(test)]
+    fn new_with_url(api_key: String, api_url: String) -> Self {
+        Self {
+            api_key,
+            client: reqwest::Client::new(),
+            api_url,
         }
     }
 
@@ -201,7 +212,7 @@ impl AnthropicProvider {
 
         let response = self
             .client
-            .post(ANTHROPIC_API_URL)
+            .post(&self.api_url)
             .headers(headers)
             .json(&request_body)
             .send()
@@ -312,3 +323,546 @@ impl AnthropicProvider {
         })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{AnthropicContent, AnthropicContentBlock, AnthropicProvider};
+    use crate::llm::types::{
+        FunctionCall, Message, Role, ToolCall, ToolDefinition, ToolFunctionDefinition,
+    };
+    use serde_json::json;
+
+    fn user_msg(content: &str) -> Message {
+        Message {
+            role: Role::User,
+            content: content.to_string(),
+            tool_calls: None,
+            tool_call_id: None,
+        }
+    }
+
+    fn system_msg(content: &str) -> Message {
+        Message {
+            role: Role::System,
+            content: content.to_string(),
+            tool_calls: None,
+            tool_call_id: None,
+        }
+    }
+
+    fn assistant_msg(content: &str) -> Message {
+        Message {
+            role: Role::Assistant,
+            content: content.to_string(),
+            tool_calls: None,
+            tool_call_id: None,
+        }
+    }
+
+    fn make_tool_def(name: &str) -> ToolDefinition {
+        ToolDefinition {
+            kind: "function".to_string(),
+            function: ToolFunctionDefinition {
+                name: name.to_string(),
+                description: format!("{name} description"),
+                parameters: json!({"type": "object", "properties": {}}),
+            },
+        }
+    }
+
+    // ── convert_tools ────────────────────────────────────────────────────────
+
+    #[test]
+    fn test_convert_tools_empty() {
+        let result = AnthropicProvider::convert_tools(&[]);
+        assert!(result.is_empty());
+    }
+
+    #[test]
+    fn test_convert_tools_single() {
+        let tool = make_tool_def("search_files");
+        let result = AnthropicProvider::convert_tools(&[tool]);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].name, "search_files");
+        assert_eq!(result[0].description, "search_files description");
+        assert_eq!(
+            result[0].input_schema,
+            json!({"type": "object", "properties": {}})
+        );
+    }
+
+    #[test]
+    fn test_convert_tools_multiple() {
+        let tools = vec![make_tool_def("read_file"), make_tool_def("write_file")];
+        let result = AnthropicProvider::convert_tools(&tools);
+        assert_eq!(result.len(), 2);
+        assert_eq!(result[0].name, "read_file");
+        assert_eq!(result[1].name, "write_file");
+    }
+
+    // ── convert_messages ─────────────────────────────────────────────────────
+
+    #[test]
+    fn test_convert_messages_user() {
+        let msgs = vec![user_msg("Hello")];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].role, "user");
+        match &result[0].content {
+            AnthropicContent::Text(t) => assert_eq!(t, "Hello"),
+            _ => panic!("Expected text content"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_system_skipped() {
+        let msgs = vec![system_msg("You are helpful"), user_msg("Hi")];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].role, "user");
+    }
+
+    #[test]
+    fn test_convert_messages_assistant_text() {
+        let msgs = vec![assistant_msg("I can help with that")];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].role, "assistant");
+        match &result[0].content {
+            AnthropicContent::Text(t) => assert_eq!(t, "I can help with that"),
+            _ => panic!("Expected text content"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_assistant_with_tool_calls_no_content() {
+        let msgs = vec![Message {
+            role: Role::Assistant,
+            content: String::new(),
+            tool_calls: Some(vec![ToolCall {
+                id: Some("toolu_abc".to_string()),
+                kind: "function".to_string(),
+                function: FunctionCall {
+                    name: "search_files".to_string(),
+                    arguments: r#"{"pattern": "*.rs"}"#.to_string(),
+                },
+            }]),
+            tool_call_id: None,
+        }];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].role, "assistant");
+        match &result[0].content {
+            AnthropicContent::Blocks(blocks) => {
+                assert_eq!(blocks.len(), 1);
+                match &blocks[0] {
+                    AnthropicContentBlock::ToolUse { id, name, .. } => {
+                        assert_eq!(id, "toolu_abc");
+                        assert_eq!(name, "search_files");
+                    }
+                    _ => panic!("Expected ToolUse block"),
+                }
+            }
+            _ => panic!("Expected blocks content"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_assistant_with_tool_calls_and_content() {
+        let msgs = vec![Message {
+            role: Role::Assistant,
+            content: "Let me search for that".to_string(),
+            tool_calls: Some(vec![ToolCall {
+                id: Some("toolu_xyz".to_string()),
+                kind: "function".to_string(),
+                function: FunctionCall {
+                    name: "read_file".to_string(),
+                    arguments: r#"{"path": "main.rs"}"#.to_string(),
+                },
+            }]),
+            tool_call_id: None,
+        }];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        assert_eq!(result.len(), 1);
+        match &result[0].content {
+            AnthropicContent::Blocks(blocks) => {
+                assert_eq!(blocks.len(), 2);
+                match &blocks[0] {
+                    AnthropicContentBlock::Text { text } => {
+                        assert_eq!(text, "Let me search for that");
+                    }
+                    _ => panic!("Expected Text block first"),
+                }
+                match &blocks[1] {
+                    AnthropicContentBlock::ToolUse { id, name, .. } => {
+                        assert_eq!(id, "toolu_xyz");
+                        assert_eq!(name, "read_file");
+                    }
+                    _ => panic!("Expected ToolUse block second"),
+                }
+            }
+            _ => panic!("Expected blocks content"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_assistant_tool_call_invalid_json_args() {
+        // Invalid JSON args fall back to {}
+        let msgs = vec![Message {
+            role: Role::Assistant,
+            content: String::new(),
+            tool_calls: Some(vec![ToolCall {
+                id: None,
+                kind: "function".to_string(),
+                function: FunctionCall {
+                    name: "my_tool".to_string(),
+                    arguments: "not valid json".to_string(),
+                },
+            }]),
+            tool_call_id: None,
+        }];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        match &result[0].content {
+            AnthropicContent::Blocks(blocks) => match &blocks[0] {
+                AnthropicContentBlock::ToolUse { input, .. } => {
+                    assert_eq!(*input, json!({}));
+                }
+                _ => panic!("Expected ToolUse block"),
+            },
+            _ => panic!("Expected blocks"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_assistant_tool_call_no_id_generates_uuid() {
+        let msgs = vec![Message {
+            role: Role::Assistant,
+            content: String::new(),
+            tool_calls: Some(vec![ToolCall {
+                id: None, // no id provided
+                kind: "function".to_string(),
+                function: FunctionCall {
+                    name: "my_tool".to_string(),
+                    arguments: "{}".to_string(),
+                },
+            }]),
+            tool_call_id: None,
+        }];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        match &result[0].content {
+            AnthropicContent::Blocks(blocks) => match &blocks[0] {
+                AnthropicContentBlock::ToolUse { id, .. } => {
+                    assert!(!id.is_empty(), "Should have generated a UUID");
+                }
+                _ => panic!("Expected ToolUse block"),
+            },
+            _ => panic!("Expected blocks"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_tool_role() {
+        let msgs = vec![Message {
+            role: Role::Tool,
+            content: "file content here".to_string(),
+            tool_calls: None,
+            tool_call_id: Some("toolu_123".to_string()),
+        }];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].role, "user");
+        match &result[0].content {
+            AnthropicContent::Blocks(blocks) => {
+                assert_eq!(blocks.len(), 1);
+                match &blocks[0] {
+                    AnthropicContentBlock::ToolResult {
+                        tool_use_id,
+                        content,
+                    } => {
+                        assert_eq!(tool_use_id, "toolu_123");
+                        assert_eq!(content, "file content here");
+                    }
+                    _ => panic!("Expected ToolResult block"),
+                }
+            }
+            _ => panic!("Expected blocks content"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_tool_role_no_id_defaults_empty() {
+        let msgs = vec![Message {
+            role: Role::Tool,
+            content: "result".to_string(),
+            tool_calls: None,
+            tool_call_id: None,
+        }];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        match &result[0].content {
+            AnthropicContent::Blocks(blocks) => match &blocks[0] {
+                AnthropicContentBlock::ToolResult { tool_use_id, .. } => {
+                    assert_eq!(tool_use_id, "");
+                }
+                _ => panic!("Expected ToolResult block"),
+            },
+            _ => panic!("Expected blocks"),
+        }
+    }
+
+    #[test]
+    fn test_convert_messages_mixed_roles() {
+        let msgs = vec![
+            system_msg("Be helpful"),
+            user_msg("What is the time?"),
+            assistant_msg("I can check that."),
+        ];
+        let result = AnthropicProvider::convert_messages(&msgs);
+        // System is skipped
+        assert_eq!(result.len(), 2);
+        assert_eq!(result[0].role, "user");
+        assert_eq!(result[1].role, "assistant");
+    }
+
+    // ── extract_system_prompt ─────────────────────────────────────────────────
+
+    #[test]
+    fn test_extract_system_prompt_no_messages() {
+        let msgs: Vec<Message> = vec![];
+        let prompt = AnthropicProvider::extract_system_prompt(&msgs);
+        assert!(prompt.is_empty());
+    }
+
+    #[test]
+    fn test_extract_system_prompt_no_system_messages() {
+        let msgs = vec![user_msg("Hello"), assistant_msg("Hi there")];
+        let prompt = AnthropicProvider::extract_system_prompt(&msgs);
+        assert!(prompt.is_empty());
+    }
+
+    #[test]
+    fn test_extract_system_prompt_single() {
+        let msgs = vec![system_msg("You are a helpful assistant"), user_msg("Hi")];
+        let prompt = AnthropicProvider::extract_system_prompt(&msgs);
+        assert_eq!(prompt, "You are a helpful assistant");
+    }
+
+    #[test]
+    fn test_extract_system_prompt_multiple_joined() {
+        let msgs = vec![
+            system_msg("First instruction"),
+            system_msg("Second instruction"),
+            user_msg("Hello"),
+        ];
+        let prompt = AnthropicProvider::extract_system_prompt(&msgs);
+        assert_eq!(prompt, "First instruction\n\nSecond instruction");
+    }
+
+    // ── chat_stream (HTTP mocked) ─────────────────────────────────────────────
+
+    #[tokio::test]
+    async fn test_chat_stream_text_response() {
+        let mut server = mockito::Server::new_async().await;
+
+        let delta1 = json!({
+            "type": "content_block_delta",
+            "delta": {"type": "text_delta", "text": "Hello"}
+        });
+        let delta2 = json!({
+            "type": "content_block_delta",
+            "delta": {"type": "text_delta", "text": " world"}
+        });
+        let body = format!("data: {delta1}\ndata: {delta2}\ndata: [DONE]\n");
+
+        let _m = server
+            .mock("POST", "/v1/messages")
+            .with_status(200)
+            .with_header("content-type", "text/event-stream")
+            .with_body(body)
+            .create_async()
+            .await;
+
+        let provider = AnthropicProvider::new_with_url(
+            "test-key".to_string(),
+            format!("{}/v1/messages", server.url()),
+        );
+        let (_tx, mut cancel_rx) = tokio::sync::watch::channel(false);
+        let mut tokens = Vec::<String>::new();
+
+        let result = provider
+            .chat_stream(
+                "claude-3-5-sonnet-20241022",
+                &[user_msg("Hello")],
+                &[],
+                &mut cancel_rx,
+                |t| tokens.push(t.to_string()),
+                |_| {},
+            )
+            .await;
+
+        assert!(result.is_ok());
+        let response = result.unwrap();
+        assert_eq!(response.content, Some("Hello world".to_string()));
+        assert!(response.tool_calls.is_none());
+        assert_eq!(tokens, vec!["Hello", " world"]);
+    }
+
+    #[tokio::test]
+    async fn test_chat_stream_error_response() {
+        let mut server = mockito::Server::new_async().await;
+
+        let _m = server
+            .mock("POST", "/v1/messages")
+            .with_status(401)
+            .with_body(r#"{"error":{"type":"authentication_error","message":"Invalid API key"}}"#)
+            .create_async()
+            .await;
+
+        let provider = AnthropicProvider::new_with_url(
+            "bad-key".to_string(),
+            format!("{}/v1/messages", server.url()),
+        );
+        let (_tx, mut cancel_rx) = tokio::sync::watch::channel(false);
+
+        let result = provider
+            .chat_stream(
+                "claude-3-5-sonnet-20241022",
+                &[user_msg("Hello")],
+                &[],
+                &mut cancel_rx,
+                |_| {},
+                |_| {},
+            )
+            .await;
+
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("401"));
+    }
+
+    #[tokio::test]
+    async fn test_chat_stream_tool_use_response() {
+        let mut server = mockito::Server::new_async().await;
+
+        let start_event = json!({
+            "type": "content_block_start",
+            "content_block": {"type": "tool_use", "id": "toolu_abc", "name": "search_files"}
+        });
+        let delta_event = json!({
+            "type": "content_block_delta",
+            "delta": {"type": "input_json_delta", "partial_json": "{}"}
+        });
+        let stop_event = json!({"type": "content_block_stop"});
+        let body = format!(
+            "data: {start_event}\ndata: {delta_event}\ndata: {stop_event}\ndata: [DONE]\n"
+        );
+
+        let _m = server
+            .mock("POST", "/v1/messages")
+            .with_status(200)
+            .with_header("content-type", "text/event-stream")
+            .with_body(body)
+            .create_async()
+            .await;
+
+        let provider = AnthropicProvider::new_with_url(
+            "test-key".to_string(),
+            format!("{}/v1/messages", server.url()),
+        );
+        let (_tx, mut cancel_rx) = tokio::sync::watch::channel(false);
+        let mut activities = Vec::<String>::new();
+
+        let result = provider
+            .chat_stream(
+                "claude-3-5-sonnet-20241022",
+                &[user_msg("Find Rust files")],
+                &[make_tool_def("search_files")],
+                &mut cancel_rx,
+                |_| {},
+                |a| activities.push(a.to_string()),
+            )
+            .await;
+
+        assert!(result.is_ok());
+        let response = result.unwrap();
+        assert!(response.content.is_none());
+        let tool_calls = response.tool_calls.expect("Expected tool calls");
+        assert_eq!(tool_calls.len(), 1);
+        assert_eq!(tool_calls[0].id, Some("toolu_abc".to_string()));
+        assert_eq!(tool_calls[0].function.name, "search_files");
+        assert_eq!(activities, vec!["search_files"]);
+    }
+
+    #[tokio::test]
+    async fn test_chat_stream_includes_system_prompt() {
+        let mut server = mockito::Server::new_async().await;
+
+        let delta = json!({
+            "type": "content_block_delta",
+            "delta": {"type": "text_delta", "text": "ok"}
+        });
+        let body = format!("data: {delta}\ndata: [DONE]\n");
+
+        let _m = server
+            .mock("POST", "/v1/messages")
+            .with_status(200)
+            .with_header("content-type", "text/event-stream")
+            .with_body(body)
+            .create_async()
+            .await;
+
+        let provider = AnthropicProvider::new_with_url(
+            "test-key".to_string(),
+            format!("{}/v1/messages", server.url()),
+        );
+        let (_tx, mut cancel_rx) = tokio::sync::watch::channel(false);
+        let messages = vec![system_msg("Be concise"), user_msg("Hello")];
+
+        let result = provider
+            .chat_stream(
+                "claude-3-5-sonnet-20241022",
+                &messages,
+                &[],
+                &mut cancel_rx,
+                |_| {},
+                |_| {},
+            )
+            .await;
+
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap().content, Some("ok".to_string()));
+    }
+
+    #[tokio::test]
+    async fn test_chat_stream_empty_response_gives_none_content() {
+        let mut server = mockito::Server::new_async().await;
+
+        let _m = server
+            .mock("POST", "/v1/messages")
+            .with_status(200)
+            .with_header("content-type", "text/event-stream")
+            .with_body("data: [DONE]\n")
+            .create_async()
+            .await;
+
+        let provider = AnthropicProvider::new_with_url(
+            "test-key".to_string(),
+            format!("{}/v1/messages", server.url()),
+        );
+        let (_tx, mut cancel_rx) = tokio::sync::watch::channel(false);
+
+        let result = provider
+            .chat_stream(
+                "claude-3-5-sonnet-20241022",
+                &[user_msg("Hello")],
+                &[],
+                &mut cancel_rx,
+                |_| {},
+                |_| {},
+            )
+            .await;
+
+        assert!(result.is_ok());
+        let response = result.unwrap();
+        assert!(response.content.is_none());
+        assert!(response.tool_calls.is_none());
+    }
+}