diff --git a/Cargo.toml b/Cargo.toml
index e385310..77d32e4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -51,6 +51,9 @@ serde_json = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
+# HTTP Client
+reqwest = { version = "0.12", features = ["json", "stream"] }
+
 # Utilities
 chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1", features = ["v4", "serde"] }
diff --git a/crates/miyabi-core/Cargo.toml b/crates/miyabi-core/Cargo.toml
index b0fd5f3..d65b827 100644
--- a/crates/miyabi-core/Cargo.toml
+++ b/crates/miyabi-core/Cargo.toml
@@ -18,3 +18,5 @@ thiserror = { workspace = true }
 tracing = { workspace = true }
 chrono = { workspace = true }
 uuid = { workspace = true }
+reqwest = { workspace = true }
+futures = { workspace = true }
diff --git a/crates/miyabi-core/src/anthropic.rs b/crates/miyabi-core/src/anthropic.rs
new file mode 100644
index 0000000..392b3f0
--- /dev/null
+++ b/crates/miyabi-core/src/anthropic.rs
@@ -0,0 +1,559 @@
+//! Anthropic API Client
+//!
+//! This module provides a client for the Anthropic Messages API
+//! with streaming support for real-time LLM responses.
+
+use futures::stream::{Stream, StreamExt};
+use reqwest::header::{HeaderMap, HeaderValue, CONTENT_TYPE};
+use serde::{Deserialize, Serialize};
+use std::pin::Pin;
+use std::time::Duration;
+use thiserror::Error;
+use tracing::{debug, error, warn};
+
+/// Anthropic API base URL
+const API_BASE_URL: &str = "https://api.anthropic.com";
+
+/// Default model to use
+pub const DEFAULT_MODEL: &str = "claude-sonnet-4-20250514";
+
+/// Maximum retry attempts for transient errors
+const MAX_RETRIES: u32 = 3;
+
+/// Base delay for exponential backoff (in milliseconds)
+const RETRY_BASE_DELAY_MS: u64 = 1000;
+
+/// Anthropic API errors
+#[derive(Error, Debug)]
+pub enum AnthropicError {
+    #[error("Authentication failed: {0}")]
+    AuthError(String),
+
+    #[error("Rate limit exceeded: retry after {retry_after_ms}ms")]
+    RateLimited { retry_after_ms: u64 },
+
+    #[error("API error ({status}): {message}")]
+    ApiError { status: u16, message: String },
+
+    #[error("Network error: {0}")]
+    NetworkError(#[from] reqwest::Error),
+
+    #[error("JSON parsing error: {0}")]
+    JsonError(#[from] serde_json::Error),
+
+    #[error("Stream error: {0}")]
+    StreamError(String),
+
+    #[error("Invalid configuration: {0}")]
+    ConfigError(String),
+}
+
+/// Result type for Anthropic operations
+pub type Result<T> = std::result::Result<T, AnthropicError>;
+
+/// Message role
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum Role {
+    User,
+    Assistant,
+}
+
+/// Content block types
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ContentBlock {
+    Text { text: String },
+    ToolUse { id: String, name: String, input: serde_json::Value },
+    ToolResult { tool_use_id: String, content: String },
+}
+
+/// A message in a conversation
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Message {
+    pub role: Role,
+    pub content: Vec<ContentBlock>,
+}
+
+impl Message {
+    /// Create a user message with text content
+    pub fn user(text: impl Into<String>) -> Self {
+        Self {
+            role: Role::User,
+            content: vec![ContentBlock::Text { text: text.into() }],
+        }
+    }
+
+    /// Create an assistant message with text content
+    pub fn assistant(text: impl Into<String>) -> Self {
+        Self {
+            role: Role::Assistant,
+            content: vec![ContentBlock::Text { text: text.into() }],
+        }
+    }
+}
+
+/// Tool definition for function calling
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Tool {
+    pub name: String,
+    pub description: String,
+    pub input_schema: serde_json::Value,
+}
+
+/// Request to the Messages API
+#[derive(Debug, Serialize)]
+pub struct MessagesRequest {
+    pub model: String,
+    pub max_tokens: u32,
+    pub messages: Vec<Message>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub system: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub tools: Option<Vec<Tool>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub temperature: Option<f32>,
+    pub stream: bool,
+}
+
+/// Stop reason for a response
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum StopReason {
+    EndTurn,
+    MaxTokens,
+    StopSequence,
+    ToolUse,
+}
+
+/// Usage statistics
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+pub struct Usage {
+    pub input_tokens: u32,
+    pub output_tokens: u32,
+}
+
+/// Response from the Messages API
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MessagesResponse {
+    pub id: String,
+    #[serde(rename = "type")]
+    pub response_type: String,
+    pub role: Role,
+    pub content: Vec<ContentBlock>,
+    pub model: String,
+    pub stop_reason: Option<StopReason>,
+    pub usage: Usage,
+}
+
+/// Streaming event types
+#[derive(Debug, Clone)]
+pub enum StreamEvent {
+    /// Message started
+    MessageStart { message: MessagesResponse },
+    /// Content block started
+    ContentBlockStart { index: usize, content_block: ContentBlock },
+    /// Text delta in content
+    ContentBlockDelta { index: usize, delta: TextDelta },
+    /// Content block finished
+    ContentBlockStop { index: usize },
+    /// Message delta (stop reason, usage)
+    MessageDelta { delta: MessageDelta, usage: Usage },
+    /// Message completed
+    MessageStop,
+    /// Ping event
+    Ping,
+    /// Error event
+    Error { error: String },
+}
+
+/// Text delta for streaming
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct TextDelta {
+    #[serde(rename = "type")]
+    pub delta_type: String,
+    pub text: String,
+}
+
+/// Message delta for streaming
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MessageDelta {
+    pub stop_reason: Option<StopReason>,
+}
+
+/// Anthropic API client
+#[derive(Clone)]
+pub struct AnthropicClient {
+    client: reqwest::Client,
+    api_key: String,
+    model: String,
+    max_tokens: u32,
+}
+
+impl AnthropicClient {
+    /// Create a new Anthropic client
+    pub fn new(api_key: impl Into<String>) -> Result<Self> {
+        let api_key = api_key.into();
+        if api_key.is_empty() {
+            return Err(AnthropicError::ConfigError(
+                "API key cannot be empty".to_string(),
+            ));
+        }
+
+        let client = reqwest::Client::builder()
+            .timeout(Duration::from_secs(300))
+            .build()
+            .map_err(AnthropicError::NetworkError)?;
+
+        Ok(Self {
+            client,
+            api_key,
+            model: DEFAULT_MODEL.to_string(),
+            max_tokens: 4096,
+        })
+    }
+
+    /// Set the model to use
+    pub fn with_model(mut self, model: impl Into<String>) -> Self {
+        self.model = model.into();
+        self
+    }
+
+    /// Set the maximum tokens
+    pub fn with_max_tokens(mut self, max_tokens: u32) -> Self {
+        self.max_tokens = max_tokens;
+        self
+    }
+
+    /// Build request headers
+    fn build_headers(&self) -> Result<HeaderMap> {
+        let mut headers = HeaderMap::new();
+        headers.insert(CONTENT_TYPE, HeaderValue::from_static("application/json"));
+        headers.insert(
+            "x-api-key",
+            HeaderValue::from_str(&self.api_key)
+                .map_err(|_| AnthropicError::ConfigError("Invalid API key format".to_string()))?,
+        );
+        headers.insert(
+            "anthropic-version",
+            HeaderValue::from_static("2023-06-01"),
+        );
+        Ok(headers)
+    }
+
+    /// Send a non-streaming message request
+    pub async fn message(
+        &self,
+        messages: Vec<Message>,
+        system: Option<String>,
+        tools: Option<Vec<Tool>>,
+        temperature: Option<f32>,
+    ) -> Result<MessagesResponse> {
+        let request = MessagesRequest {
+            model: self.model.clone(),
+            max_tokens: self.max_tokens,
+            messages,
+            system,
+            tools,
+            temperature,
+            stream: false,
+        };
+
+        self.send_with_retry(&request).await
+    }
+
+    /// Send request with retry logic
+    async fn send_with_retry(&self, request: &MessagesRequest) -> Result<MessagesResponse> {
+        let mut last_error = None;
+
+        for attempt in 0..MAX_RETRIES {
+            match self.send_request(request).await {
+                Ok(response) => return Ok(response),
+                Err(e) => {
+                    match &e {
+                        AnthropicError::RateLimited { retry_after_ms } => {
+                            if attempt < MAX_RETRIES - 1 {
+                                warn!("Rate limited, retrying after {}ms", retry_after_ms);
+                                tokio::time::sleep(Duration::from_millis(*retry_after_ms)).await;
+                            }
+                        }
+                        AnthropicError::NetworkError(_) => {
+                            if attempt < MAX_RETRIES - 1 {
+                                let delay = RETRY_BASE_DELAY_MS * 2u64.pow(attempt);
+                                warn!("Network error, retrying after {}ms", delay);
+                                tokio::time::sleep(Duration::from_millis(delay)).await;
+                            }
+                        }
+                        _ => return Err(e),
+                    }
+                    last_error = Some(e);
+                }
+            }
+        }
+
+        Err(last_error.unwrap_or(AnthropicError::StreamError(
+            "Max retries exceeded".to_string()
+        )))
+    }
+
+    /// Send a single request
+    async fn send_request(&self, request: &MessagesRequest) -> Result<MessagesResponse> {
+        let url = format!("{}/v1/messages", API_BASE_URL);
+        let headers = self.build_headers()?;
+
+        debug!("Sending request to {}", url);
+
+        let response = self
+            .client
+            .post(&url)
+            .headers(headers)
+            .json(request)
+            .send()
+            .await?;
+
+        let status = response.status();
+
+        if !status.is_success() {
+            return Err(self.handle_error_response(response).await);
+        }
+
+        let body = response.json().await?;
+        Ok(body)
+    }
+
+    /// Handle error response
+    async fn handle_error_response(&self, response: reqwest::Response) -> AnthropicError {
+        let status = response.status().as_u16();
+
+        // Check for rate limit headers
+        if status == 429 {
+            let retry_after = response
+                .headers()
+                .get("retry-after")
+                .and_then(|v| v.to_str().ok())
+                .and_then(|s| s.parse::<u64>().ok())
+                .unwrap_or(60)
+                * 1000;
+
+            return AnthropicError::RateLimited {
+                retry_after_ms: retry_after,
+            };
+        }
+
+        // Try to parse error body
+        let message = match response.text().await {
+            Ok(body) => {
+                if let Ok(json) = serde_json::from_str::<serde_json::Value>(&body) {
+                    json.get("error")
+                        .and_then(|e| e.get("message"))
+                        .and_then(|m| m.as_str())
+                        .unwrap_or(&body)
+                        .to_string()
+                } else {
+                    body
+                }
+            }
+            Err(_) => "Unknown error".to_string(),
+        };
+
+        match status {
+            401 => AnthropicError::AuthError(message),
+            _ => AnthropicError::ApiError { status, message },
+        }
+    }
+
+    /// Send a streaming message request
+    pub async fn message_stream(
+        &self,
+        messages: Vec<Message>,
+        system: Option<String>,
+        tools: Option<Vec<Tool>>,
+        temperature: Option<f32>,
+    ) -> Result<Pin<Box<dyn Stream<Item = Result<StreamEvent>> + Send>>> {
+        let request = MessagesRequest {
+            model: self.model.clone(),
+            max_tokens: self.max_tokens,
+            messages,
+            system,
+            tools,
+            temperature,
+            stream: true,
+        };
+
+        let url = format!("{}/v1/messages", API_BASE_URL);
+        let headers = self.build_headers()?;
+
+        debug!("Starting stream request to {}", url);
+
+        let response = self
+            .client
+            .post(&url)
+            .headers(headers)
+            .json(&request)
+            .send()
+            .await?;
+
+        let status = response.status();
+
+        if !status.is_success() {
+            return Err(self.handle_error_response(response).await);
+        }
+
+        let stream = response.bytes_stream();
+
+        Ok(Box::pin(stream.scan(String::new(), |buffer, chunk| {
+            let result = match chunk {
+                Ok(bytes) => {
+                    buffer.push_str(&String::from_utf8_lossy(&bytes));
+
+                    let mut events = Vec::new();
+
+                    // Parse SSE events from buffer
+                    while let Some(event_end) = buffer.find("\n\n") {
+                        let event_data = buffer[..event_end].to_string();
+                        *buffer = buffer[event_end + 2..].to_string();
+
+                        if let Some(event) = parse_sse_event(&event_data) {
+                            events.push(Ok(event));
+                        }
+                    }
+
+                    Some(futures::stream::iter(events))
+                }
+                Err(e) => Some(futures::stream::iter(vec![Err(AnthropicError::NetworkError(e))])),
+            };
+            async move { result }
+        }).flatten()))
+    }
+}
+
+/// Parse a single SSE event
+fn parse_sse_event(event_data: &str) -> Option<StreamEvent> {
+    let mut event_type = None;
+    let mut data = None;
+
+    for line in event_data.lines() {
+        if let Some(value) = line.strip_prefix("event: ") {
+            event_type = Some(value.trim().to_string());
+        } else if let Some(value) = line.strip_prefix("data: ") {
+            data = Some(value.trim().to_string());
+        }
+    }
+
+    let event_type = event_type?;
+    let data = data?;
+
+    match event_type.as_str() {
+        "message_start" => {
+            let parsed: serde_json::Value = serde_json::from_str(&data).ok()?;
+            let message: MessagesResponse = serde_json::from_value(parsed.get("message")?.clone()).ok()?;
+            Some(StreamEvent::MessageStart { message })
+        }
+        "content_block_start" => {
+            let parsed: serde_json::Value = serde_json::from_str(&data).ok()?;
+            let index = parsed.get("index")?.as_u64()? as usize;
+            let content_block: ContentBlock = serde_json::from_value(parsed.get("content_block")?.clone()).ok()?;
+            Some(StreamEvent::ContentBlockStart { index, content_block })
+        }
+        "content_block_delta" => {
+            let parsed: serde_json::Value = serde_json::from_str(&data).ok()?;
+            let index = parsed.get("index")?.as_u64()? as usize;
+            let delta: TextDelta = serde_json::from_value(parsed.get("delta")?.clone()).ok()?;
+            Some(StreamEvent::ContentBlockDelta { index, delta })
+        }
+        "content_block_stop" => {
+            let parsed: serde_json::Value = serde_json::from_str(&data).ok()?;
+            let index = parsed.get("index")?.as_u64()? as usize;
+            Some(StreamEvent::ContentBlockStop { index })
+        }
+        "message_delta" => {
+            let parsed: serde_json::Value = serde_json::from_str(&data).ok()?;
+            let delta: MessageDelta = serde_json::from_value(parsed.get("delta")?.clone()).ok()?;
+            let usage: Usage = serde_json::from_value(parsed.get("usage")?.clone()).ok()?;
+            Some(StreamEvent::MessageDelta { delta, usage })
+        }
+        "message_stop" => Some(StreamEvent::MessageStop),
+        "ping" => Some(StreamEvent::Ping),
+        "error" => {
+            let parsed: serde_json::Value = serde_json::from_str(&data).ok()?;
+            let error = parsed.get("error")?.get("message")?.as_str()?.to_string();
+            Some(StreamEvent::Error { error })
+        }
+        _ => {
+            debug!("Unknown event type: {}", event_type);
+            None
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_client_creation() {
+        let client = AnthropicClient::new("test-api-key");
+        assert!(client.is_ok());
+    }
+
+    #[test]
+    fn test_client_empty_key() {
+        let client = AnthropicClient::new("");
+        assert!(matches!(client, Err(AnthropicError::ConfigError(_))));
+    }
+
+    #[test]
+    fn test_message_creation() {
+        let msg = Message::user("Hello");
+        assert_eq!(msg.role, Role::User);
+        assert_eq!(msg.content.len(), 1);
+    }
+
+    #[test]
+    fn test_client_builder() {
+        let client = AnthropicClient::new("test-key")
+            .unwrap()
+            .with_model("claude-3-opus")
+            .with_max_tokens(8192);
+
+        assert_eq!(client.model, "claude-3-opus");
+        assert_eq!(client.max_tokens, 8192);
+    }
+
+    #[test]
+    fn test_parse_sse_text_delta() {
+        let event_data = "event: content_block_delta\ndata: {\"type\":\"content_block_delta\",\"index\":0,\"delta\":{\"type\":\"text_delta\",\"text\":\"Hello\"}}";
+        let event = parse_sse_event(event_data);
+
+        assert!(matches!(event, Some(StreamEvent::ContentBlockDelta { .. })));
+    }
+
+    #[test]
+    fn test_parse_sse_message_stop() {
+        let event_data = "event: message_stop\ndata: {}";
+        let event = parse_sse_event(event_data);
+
+        assert!(matches!(event, Some(StreamEvent::MessageStop)));
+    }
+
+    #[test]
+    fn test_role_serialization() {
+        let user = Role::User;
+        let json = serde_json::to_string(&user).unwrap();
+        assert_eq!(json, "\"user\"");
+
+        let assistant = Role::Assistant;
+        let json = serde_json::to_string(&assistant).unwrap();
+        assert_eq!(json, "\"assistant\"");
+    }
+
+    #[test]
+    fn test_content_block_text() {
+        let block = ContentBlock::Text {
+            text: "Hello".to_string(),
+        };
+        let json = serde_json::to_string(&block).unwrap();
+        assert!(json.contains("\"type\":\"text\""));
+        assert!(json.contains("\"text\":\"Hello\""));
+    }
+}
diff --git a/crates/miyabi-core/src/lib.rs b/crates/miyabi-core/src/lib.rs
index e7ed082..27440bf 100644
--- a/crates/miyabi-core/src/lib.rs
+++ b/crates/miyabi-core/src/lib.rs
@@ -4,6 +4,11 @@
 
 pub mod error;
 pub mod types;
+pub mod anthropic;
 
 pub use error::Error;
 pub use types::*;
+pub use anthropic::{
+    AnthropicClient, AnthropicError, Message, Role, ContentBlock,
+    Tool, MessagesRequest, MessagesResponse, StreamEvent, StopReason, Usage,
+};