diff --git a/src/error.rs b/src/error.rs
index ab34dcb..36c0c0e 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -40,3 +40,48 @@ impl From<LinesCodecError> for OllamaError {
         Self::LinesCodecError(value)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn display_response_parse_error() {
+        let err: Result<i32, _> = serde_json::from_str("not json");
+        let ollama_err = OllamaError::from(err.unwrap_err());
+        let display = format!("{}", ollama_err);
+        assert!(display.starts_with("Response parse error:"));
+        assert!(!display.ends_with('\n'));
+    }
+
+    #[test]
+    fn display_lines_codec_error() {
+        let err = LinesCodecError::MaxLineLengthExceeded;
+        let ollama_err = OllamaError::from(err);
+        let display = format!("{}", ollama_err);
+        assert!(display.starts_with("Lines codec error:"));
+        assert!(!display.ends_with('\n'));
+    }
+
+    #[test]
+    fn from_serde_json_error() {
+        let err: Result<i32, _> = serde_json::from_str("not json");
+        let ollama_err = OllamaError::from(err.unwrap_err());
+        assert!(matches!(ollama_err, OllamaError::ResponseParseError(_)));
+    }
+
+    #[test]
+    fn from_lines_codec_error() {
+        let err = LinesCodecError::MaxLineLengthExceeded;
+        let ollama_err = OllamaError::from(err);
+        assert!(matches!(ollama_err, OllamaError::LinesCodecError(_)));
+    }
+
+    #[test]
+    fn error_trait_is_implemented() {
+        let err: Result<i32, _> = serde_json::from_str("not json");
+        let ollama_err = OllamaError::from(err.unwrap_err());
+        // Verify it implements std::error::Error by using it as a trait object
+        let _: &dyn Error = &ollama_err;
+    }
+}
diff --git a/src/types/chat.rs b/src/types/chat.rs
index db1f16e..957423e 100644
--- a/src/types/chat.rs
+++ b/src/types/chat.rs
@@ -172,3 +172,172 @@ pub struct ToolCallFunction {
     pub arguments: Value,
     pub index: usize,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn role_serializes_lowercase() {
+        assert_eq!(serde_json::to_value(&Role::User).unwrap(), json!("user"));
+        assert_eq!(
+            serde_json::to_value(&Role::System).unwrap(),
+            json!("system")
+        );
+        assert_eq!(
+            serde_json::to_value(&Role::Assistant).unwrap(),
+            json!("assistant")
+        );
+        assert_eq!(serde_json::to_value(&Role::Tool).unwrap(), json!("tool"));
+    }
+
+    #[test]
+    fn role_deserializes_lowercase() {
+        let role: Role = serde_json::from_value(json!("user")).unwrap();
+        assert!(matches!(role, Role::User));
+    }
+
+    #[test]
+    fn message_system_constructor() {
+        let msg = Message::system("you are helpful");
+        assert_eq!(msg.content, "you are helpful");
+        assert!(matches!(msg.role, Role::System));
+        assert!(msg.tool_calls.is_empty());
+    }
+
+    #[test]
+    fn message_user_constructor() {
+        let msg = Message::user("hello");
+        assert_eq!(msg.content, "hello");
+        assert!(matches!(msg.role, Role::User));
+    }
+
+    #[test]
+    fn message_tool_response_constructor() {
+        let value = json!({"temperature": 22.0});
+        let msg = Message::tool_response(&value).unwrap();
+        assert!(matches!(msg.role, Role::Tool));
+        assert_eq!(msg.content, serde_json::to_string(&value).unwrap());
+    }
+
+    #[test]
+    fn message_skips_empty_tool_calls() {
+        let msg = Message::user("hello");
+        let json = serde_json::to_value(&msg).unwrap();
+        assert!(!json.as_object().unwrap().contains_key("tool_calls"));
+    }
+
+    #[test]
+    fn message_deserializes_without_tool_calls() {
+        let json = json!({"content": "hi", "role": "user"});
+        let msg: Message = serde_json::from_value(json).unwrap();
+        assert_eq!(msg.content, "hi");
+        assert!(msg.tool_calls.is_empty());
+    }
+
+    #[test]
+    fn chat_request_always_serializes_messages() {
+        let request = ChatRequest::builder("llama3").build();
+        let json = serde_json::to_value(&request).unwrap();
+        let obj = json.as_object().unwrap();
+        assert!(obj.contains_key("messages"));
+        assert_eq!(obj["messages"], json!([]));
+    }
+
+    #[test]
+    fn chat_request_skips_optional_fields() {
+        let request = ChatRequest::builder("llama3").build();
+        let json = serde_json::to_value(&request).unwrap();
+        let obj = json.as_object().unwrap();
+        assert!(!obj.contains_key("stream"));
+        assert!(!obj.contains_key("options"));
+        assert!(!obj.contains_key("tools"));
+        assert!(!obj.contains_key("format"));
+        assert!(!obj.contains_key("think"));
+    }
+
+    #[test]
+    fn chat_request_builder_with_messages() {
+        let messages = vec![Message::system("be helpful"), Message::user("hello")];
+        let request = ChatRequest::builder("llama3")
+            .messages(messages)
+            .stream(false)
+            .build();
+
+        assert_eq!(request.model, "llama3");
+        assert_eq!(request.messages.len(), 2);
+        assert_eq!(request.stream, Some(false));
+    }
+
+    #[test]
+    fn tool_type_serializes_as_type_field() {
+        let tool = Tool {
+            tool_type: ToolType::Function,
+            function: Function {
+                name: "get_weather".to_string(),
+                description: "Get weather".to_string(),
+                parameters: json!({"type": "object"}),
+            },
+        };
+        let json = serde_json::to_value(&tool).unwrap();
+        let obj = json.as_object().unwrap();
+        assert!(obj.contains_key("type"));
+        assert!(!obj.contains_key("tool_type"));
+        assert_eq!(obj["type"], json!("function"));
+    }
+
+    #[test]
+    fn tool_type_deserializes_from_type_field() {
+        let json = json!({
+            "type": "function",
+            "function": {
+                "name": "get_weather",
+                "description": "Get weather",
+                "parameters": {"type": "object"}
+            }
+        });
+        let tool: Tool = serde_json::from_value(json).unwrap();
+        assert!(matches!(tool.tool_type, ToolType::Function));
+        assert_eq!(tool.function.name, "get_weather");
+    }
+
+    #[test]
+    fn chat_response_deserialize() {
+        let json = json!({
+            "model": "llama3",
+            "created_at": "2024-01-01T00:00:00Z",
+            "message": {"content": "Hello!", "role": "assistant"},
+            "done": false
+        });
+        let response: ChatResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.model, "llama3");
+        assert_eq!(response.message.content, "Hello!");
+        assert!(matches!(response.message.role, Role::Assistant));
+        assert!(!response.done);
+    }
+
+    #[test]
+    fn chat_response_with_tool_calls() {
+        let json = json!({
+            "model": "llama3",
+            "created_at": "2024-01-01T00:00:00Z",
+            "message": {
+                "content": "",
+                "role": "assistant",
+                "tool_calls": [{
+                    "function": {
+                        "name": "get_weather",
+                        "arguments": {"city": "Paris"},
+                        "index": 0
+                    }
+                }]
+            },
+            "done": true
+        });
+        let response: ChatResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.message.tool_calls.len(), 1);
+        assert_eq!(response.message.tool_calls[0].function.name, "get_weather");
+        assert_eq!(response.message.tool_calls[0].function.index, 0);
+    }
+}
diff --git a/src/types/common.rs b/src/types/common.rs
index 39c7598..1cf52cc 100644
--- a/src/types/common.rs
+++ b/src/types/common.rs
@@ -123,3 +123,171 @@ pub enum Stop {
     Single(String),
     Multiple(Vec<String>),
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn think_bool_true() {
+        let think: Think = serde_json::from_value(json!(true)).unwrap();
+        assert!(matches!(think, Think::Bool(true)));
+    }
+
+    #[test]
+    fn think_bool_false() {
+        let think: Think = serde_json::from_value(json!(false)).unwrap();
+        assert!(matches!(think, Think::Bool(false)));
+    }
+
+    #[test]
+    fn think_level_high() {
+        let think: Think = serde_json::from_value(json!("high")).unwrap();
+        assert!(matches!(think, Think::Level(ThinkLevel::High)));
+    }
+
+    #[test]
+    fn think_level_medium() {
+        let think: Think = serde_json::from_value(json!("medium")).unwrap();
+        assert!(matches!(think, Think::Level(ThinkLevel::Medium)));
+    }
+
+    #[test]
+    fn think_level_low() {
+        let think: Think = serde_json::from_value(json!("low")).unwrap();
+        assert!(matches!(think, Think::Level(ThinkLevel::Low)));
+    }
+
+    #[test]
+    fn think_bool_round_trip() {
+        let think = Think::Bool(true);
+        let json = serde_json::to_value(&think).unwrap();
+        assert_eq!(json, json!(true));
+    }
+
+    #[test]
+    fn think_level_round_trip() {
+        let think = Think::Level(ThinkLevel::High);
+        let json = serde_json::to_value(&think).unwrap();
+        assert_eq!(json, json!("high"));
+    }
+
+    #[test]
+    fn stop_single() {
+        let stop: Stop = serde_json::from_value(json!("end")).unwrap();
+        assert!(matches!(stop, Stop::Single(s) if s == "end"));
+    }
+
+    #[test]
+    fn stop_multiple() {
+        let stop: Stop = serde_json::from_value(json!(["end", "stop"])).unwrap();
+        match stop {
+            Stop::Multiple(v) => assert_eq!(v, vec!["end", "stop"]),
+            _ => panic!("expected Stop::Multiple"),
+        }
+    }
+
+    #[test]
+    fn stop_single_round_trip() {
+        let stop = Stop::Single("end".to_string());
+        let json = serde_json::to_value(&stop).unwrap();
+        assert_eq!(json, json!("end"));
+    }
+
+    #[test]
+    fn stop_multiple_round_trip() {
+        let stop = Stop::Multiple(vec!["end".to_string(), "stop".to_string()]);
+        let json = serde_json::to_value(&stop).unwrap();
+        assert_eq!(json, json!(["end", "stop"]));
+    }
+
+    #[test]
+    fn options_default_serializes_empty() {
+        let options = Options::default();
+        let json = serde_json::to_value(&options).unwrap();
+        assert_eq!(json, json!({}));
+    }
+
+    #[test]
+    fn options_skips_none_fields() {
+        let options = Options::builder().seed(42).temperature(0.5).build();
+        let json = serde_json::to_value(&options).unwrap();
+        assert_eq!(json, json!({"seed": 42, "temperature": 0.5}));
+        assert!(!json.as_object().unwrap().contains_key("top_k"));
+    }
+
+    #[test]
+    fn options_builder_all_fields() {
+        let options = Options::builder()
+            .seed(42)
+            .temperature(0.7)
+            .top_k(40)
+            .top_p(0.9)
+            .min_p(0.05)
+            .stop(Stop::Single("end".to_string()))
+            .num_ctx(4096)
+            .num_predict(128)
+            .build();
+
+        assert_eq!(options.seed, Some(42));
+        assert_eq!(options.temperature, Some(0.7));
+        assert_eq!(options.top_k, Some(40));
+        assert_eq!(options.top_p, Some(0.9));
+        assert_eq!(options.min_p, Some(0.05));
+        assert!(options.stop.is_some());
+        assert_eq!(options.num_ctx, Some(4096));
+        assert_eq!(options.num_predict, Some(128));
+    }
+
+    #[test]
+    fn options_round_trip() {
+        let json = json!({
+            "seed": 42,
+            "temperature": 0.5,
+            "top_k": 40,
+            "num_ctx": 4096
+        });
+        let options: Options = serde_json::from_value(json.clone()).unwrap();
+        assert_eq!(options.seed, Some(42));
+        assert_eq!(options.temperature, Some(0.5));
+        assert_eq!(options.top_k, Some(40));
+        assert_eq!(options.num_ctx, Some(4096));
+        assert_eq!(options.top_p, None);
+
+        let reserialized = serde_json::to_value(&options).unwrap();
+        assert_eq!(reserialized, json);
+    }
+
+    #[test]
+    fn model_details_round_trip() {
+        let json = json!({
+            "format": "gguf",
+            "family": "llama",
+            "families": ["llama", "clip"],
+            "parameter_size": "8B",
+            "quantization_level": "Q4_0"
+        });
+        let details: ModelDetails = serde_json::from_value(json).unwrap();
+        assert_eq!(details.format, "gguf");
+        assert_eq!(details.family, "llama");
+        assert_eq!(
+            details.families,
+            Some(vec!["llama".to_string(), "clip".to_string()])
+        );
+        assert_eq!(details.parameter_size, "8B");
+        assert_eq!(details.quantization_level, "Q4_0");
+    }
+
+    #[test]
+    fn model_details_without_families() {
+        let json = json!({
+            "format": "gguf",
+            "family": "llama",
+            "parameter_size": "8B",
+            "quantization_level": "Q4_0"
+        });
+        let details: ModelDetails = serde_json::from_value(json).unwrap();
+        assert_eq!(details.families, None);
+    }
+}
diff --git a/src/types/generate.rs b/src/types/generate.rs
index a943f5a..2602953 100644
--- a/src/types/generate.rs
+++ b/src/types/generate.rs
@@ -154,3 +154,122 @@ pub struct GenerateResponse {
     /// Time spent generating tokens in nanoseconds
     pub eval_duration: Option<u64>,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn builder_minimal() {
+        let request = GenerateRequest::builder("llama3").build();
+        assert_eq!(request.model, "llama3");
+        assert!(request.prompt.is_none());
+        assert!(request.system.is_none());
+        assert!(request.images.is_empty());
+    }
+
+    #[test]
+    fn builder_with_all_fields() {
+        let request = GenerateRequest::builder("llama3")
+            .prompt("hello")
+            .system_prompt("you are helpful")
+            .suffix("end".to_string())
+            .stream(false)
+            .images(vec!["base64data".to_string()])
+            .format(json!("json"))
+            .think(Think::Bool(true))
+            .options(Options::builder().seed(42).build())
+            .build();
+
+        assert_eq!(request.model, "llama3");
+        assert_eq!(request.prompt, Some("hello".to_string()));
+        assert_eq!(request.system, Some("you are helpful".to_string()));
+        assert_eq!(request.suffix, Some("end".to_string()));
+        assert_eq!(request.stream, Some(false));
+        assert_eq!(request.images, vec!["base64data".to_string()]);
+        assert!(request.format.is_some());
+        assert!(request.think.is_some());
+        assert!(request.options.is_some());
+    }
+
+    #[test]
+    fn request_skips_none_fields() {
+        let request = GenerateRequest::builder("llama3").prompt("hello").build();
+        let json = serde_json::to_value(&request).unwrap();
+        let obj = json.as_object().unwrap();
+
+        assert!(obj.contains_key("model"));
+        assert!(obj.contains_key("prompt"));
+        assert!(!obj.contains_key("suffix"));
+        assert!(!obj.contains_key("system"));
+        assert!(!obj.contains_key("stream"));
+        assert!(!obj.contains_key("images"));
+        assert!(!obj.contains_key("format"));
+        assert!(!obj.contains_key("think"));
+        assert!(!obj.contains_key("options"));
+    }
+
+    #[test]
+    fn request_includes_images_when_nonempty() {
+        let request = GenerateRequest::builder("llama3")
+            .images(vec!["abc".to_string()])
+            .build();
+        let json = serde_json::to_value(&request).unwrap();
+        assert!(json.as_object().unwrap().contains_key("images"));
+    }
+
+    #[test]
+    fn response_deserialize_streaming_chunk() {
+        let json = json!({
+            "model": "llama3",
+            "created_at": "2024-01-01T00:00:00Z",
+            "response": "Hello",
+            "done": false
+        });
+        let response: GenerateResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.model, "llama3");
+        assert_eq!(response.response, "Hello");
+        assert!(!response.done);
+        assert!(response.done_reason.is_none());
+        assert!(response.total_duration.is_none());
+    }
+
+    #[test]
+    fn response_deserialize_final_chunk() {
+        let json = json!({
+            "model": "llama3",
+            "created_at": "2024-01-01T00:00:00Z",
+            "response": "",
+            "done": true,
+            "done_reason": "stop",
+            "total_duration": 5000000000u64,
+            "load_duration": 1000000000u64,
+            "prompt_eval_count": 10,
+            "prompt_eval_duration": 500000000u64,
+            "eval_count": 50,
+            "eval_duration": 3500000000u64
+        });
+        let response: GenerateResponse = serde_json::from_value(json).unwrap();
+        assert!(response.done);
+        assert_eq!(response.done_reason, Some("stop".to_string()));
+        assert_eq!(response.total_duration, Some(5_000_000_000));
+        assert_eq!(response.eval_count, Some(50));
+    }
+
+    #[test]
+    fn response_deserialize_with_thinking() {
+        let json = json!({
+            "model": "llama3",
+            "created_at": "2024-01-01T00:00:00Z",
+            "response": "The answer is 42.",
+            "thinking": "Let me think about this...",
+            "done": true
+        });
+        let response: GenerateResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(
+            response.thinking,
+            Some("Let me think about this...".to_string())
+        );
+    }
+}
diff --git a/src/types/ps.rs b/src/types/ps.rs
index b5f735c..fe629c7 100644
--- a/src/types/ps.rs
+++ b/src/types/ps.rs
@@ -18,3 +18,42 @@ pub struct RunningModel {
     pub size_vram: u64,
     pub context_length: u32,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn deserialize_ps_response() {
+        let json = json!({
+            "models": [{
+                "name": "llama3:latest",
+                "model": "llama3:latest",
+                "size": 4_700_000_000u64,
+                "digest": "abc123",
+                "details": {
+                    "format": "gguf",
+                    "family": "llama",
+                    "parameter_size": "8B",
+                    "quantization_level": "Q4_0"
+                },
+                "expires_at": "2024-01-01T01:00:00Z",
+                "size_vram": 4_700_000_000u64,
+                "context_length": 8192
+            }]
+        });
+        let response: PsResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.models.len(), 1);
+        assert_eq!(response.models[0].name, "llama3:latest");
+        assert_eq!(response.models[0].size_vram, 4_700_000_000);
+        assert_eq!(response.models[0].context_length, 8192);
+    }
+
+    #[test]
+    fn deserialize_empty_models() {
+        let json = json!({"models": []});
+        let response: PsResponse = serde_json::from_value(json).unwrap();
+        assert!(response.models.is_empty());
+    }
+}
diff --git a/src/types/pull.rs b/src/types/pull.rs
index e90861a..df732fc 100644
--- a/src/types/pull.rs
+++ b/src/types/pull.rs
@@ -45,3 +45,51 @@ impl PullRequestBuilder {
 pub struct PullResponse {
     pub status: String,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn builder_minimal() {
+        let request = PullRequest::builder("llama3").build();
+        assert_eq!(request.model, "llama3");
+        assert!(request.insecure.is_none());
+        assert!(request.stream.is_none());
+    }
+
+    #[test]
+    fn builder_with_options() {
+        let request = PullRequest::builder("llama3")
+            .stream(true)
+            .insecure(false)
+            .build();
+        assert_eq!(request.stream, Some(true));
+        assert_eq!(request.insecure, Some(false));
+    }
+
+    #[test]
+    fn request_skips_none_fields() {
+        let request = PullRequest::builder("llama3").build();
+        let json = serde_json::to_value(&request).unwrap();
+        let obj = json.as_object().unwrap();
+        assert!(obj.contains_key("model"));
+        assert!(!obj.contains_key("insecure"));
+        assert!(!obj.contains_key("stream"));
+    }
+
+    #[test]
+    fn request_includes_set_fields() {
+        let request = PullRequest::builder("llama3").stream(true).build();
+        let json = serde_json::to_value(&request).unwrap();
+        assert_eq!(json, json!({"model": "llama3", "stream": true}));
+    }
+
+    #[test]
+    fn response_deserialize() {
+        let json = json!({"status": "pulling manifest"});
+        let response: PullResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.status, "pulling manifest");
+    }
+}
diff --git a/src/types/tags.rs b/src/types/tags.rs
index 90e209d..4c861de 100644
--- a/src/types/tags.rs
+++ b/src/types/tags.rs
@@ -16,3 +16,40 @@ pub struct Model {
     pub digest: String,
     pub details: ModelDetails,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn deserialize_tags_response() {
+        let json = json!({
+            "models": [{
+                "name": "llama3:latest",
+                "model": "llama3:latest",
+                "modified_at": "2024-01-01T00:00:00Z",
+                "size": 4_700_000_000u64,
+                "digest": "abc123",
+                "details": {
+                    "format": "gguf",
+                    "family": "llama",
+                    "parameter_size": "8B",
+                    "quantization_level": "Q4_0"
+                }
+            }]
+        });
+        let response: TagsResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.models.len(), 1);
+        assert_eq!(response.models[0].name, "llama3:latest");
+        assert_eq!(response.models[0].size, 4_700_000_000);
+        assert_eq!(response.models[0].details.family, "llama");
+    }
+
+    #[test]
+    fn deserialize_empty_models() {
+        let json = json!({"models": []});
+        let response: TagsResponse = serde_json::from_value(json).unwrap();
+        assert!(response.models.is_empty());
+    }
+}
diff --git a/src/types/version.rs b/src/types/version.rs
index 65730e3..ec92427 100644
--- a/src/types/version.rs
+++ b/src/types/version.rs
@@ -4,3 +4,25 @@ use serde::{Deserialize, Serialize};
 pub struct VersionResponse {
     pub version: String,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn deserialize() {
+        let json = json!({"version": "0.6.2"});
+        let response: VersionResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.version, "0.6.2");
+    }
+
+    #[test]
+    fn round_trip() {
+        let response = VersionResponse {
+            version: "0.6.2".to_string(),
+        };
+        let json = serde_json::to_value(&response).unwrap();
+        assert_eq!(json, json!({"version": "0.6.2"}));
+    }
+}