diff --git a/src/lib.rs b/src/lib.rs
index 8e3c9ff..9ca3cd8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -4,7 +4,7 @@ pub mod cli;
 // pub mod commands;
 // pub mod db;
 pub mod error;
-// pub mod models;
+pub mod models;
 // pub mod output;
 pub mod util;
 
diff --git a/src/models/mod.rs b/src/models/mod.rs
new file mode 100644
index 0000000..721e2c2
--- /dev/null
+++ b/src/models/mod.rs
@@ -0,0 +1,3 @@
+//! Data models for Claude Code session JSONL files.
+pub mod session;
+pub mod stats;
diff --git a/src/models/session.rs b/src/models/session.rs
new file mode 100644
index 0000000..cdce051
--- /dev/null
+++ b/src/models/session.rs
@@ -0,0 +1,253 @@
+//! Rust types for deserializing Claude Code JSONL session files.
+//!
+//! These types are intentionally permissive — real session files contain many
+//! undocumented fields, so `RawEntry` uses `#[serde(flatten)]` to capture
+//! unknown fields rather than failing deserialization.
+
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+/// Raw deserialized line from a JSONL session file.
+///
+/// Uses permissive deserialization — NO `deny_unknown_fields`.
+/// All fields are optional because different entry types have different fields.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct RawEntry {
+    /// Entry type: `"user"`, `"assistant"`, `"system"`, `"progress"`,
+    /// `"file-history-snapshot"`, etc.
+    #[serde(rename = "type")]
+    pub entry_type: Option<String>,
+    /// UUID of this session.
+    pub session_id: Option<String>,
+    /// UUID of the parent agent, if this is a sub-agent run.
+    pub parent_session_id: Option<String>,
+    /// The message payload (present on user/assistant entries).
+    pub message: Option<Message>,
+    /// System message payload.
+    pub system_message: Option<SystemMessage>,
+    /// Current working directory (present on first system entry).
+    pub cwd: Option<String>,
+    /// Timestamp string (ISO 8601).
+    pub timestamp: Option<String>,
+    /// Unix milliseconds of the conversation turn duration.
+    pub duration_ms: Option<u64>,
+    /// Raw JSON for any fields this struct doesn't explicitly model.
+    #[serde(flatten)]
+    pub extra: std::collections::HashMap<String, Value>,
+}
+
+/// A message within a session entry (user or assistant role).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Message {
+    /// Role: `"user"` or `"assistant"`.
+    pub role: Option<String>,
+    /// Content: either a plain string or an array of content blocks.
+    pub content: Option<MessageContent>,
+    /// Token usage statistics (present on assistant messages).
+    pub usage: Option<Usage>,
+    /// Model identifier (e.g. `"claude-opus-4-5-20251001"`).
+    pub model: Option<String>,
+    /// Stop reason from the API (e.g. `"end_turn"`, `"tool_use"`).
+    pub stop_reason: Option<String>,
+}
+
+/// Message content: either a plain text string or an array of typed blocks.
+///
+/// Untagged: tries `Blocks` first, falls back to `Text`.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum MessageContent {
+    /// Array of structured content blocks.
+    Blocks(Vec<ContentBlock>),
+    /// Plain text string content.
+    Text(String),
+}
+
+/// A single content block within a message.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ContentBlock {
+    /// Plain text output.
+    Text {
+        /// The text content.
+        text: String,
+    },
+    /// Extended thinking block.
+    Thinking {
+        /// The thinking content.
+        thinking: String,
+    },
+    /// A tool invocation.
+    ToolUse {
+        /// Unique ID for this tool call.
+        id: String,
+        /// Tool name (e.g. `"Bash"`, `"Read"`, `"Write"`).
+        name: String,
+        /// Tool call arguments as raw JSON.
+        input: Value,
+    },
+    /// Result of a tool invocation.
+    ToolResult {
+        /// The `tool_use` id this result corresponds to.
+        tool_use_id: String,
+        /// Result content (string or nested blocks).
+        content: Option<Value>,
+        /// `true` if the tool returned an error.
+        is_error: Option<bool>,
+    },
+    /// An image block.
+    Image {
+        /// Image source details.
+        source: Option<Value>,
+    },
+    /// Catch-all for unknown block types encountered in real data.
+    #[serde(other)]
+    Unknown,
+}
+
+/// Token usage for an assistant message.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct Usage {
+    /// Tokens in the input prompt.
+    pub input_tokens: Option<u64>,
+    /// Tokens in the generated output.
+    pub output_tokens: Option<u64>,
+    /// Tokens read from the cache.
+    pub cache_read_input_tokens: Option<u64>,
+    /// Tokens written to the cache.
+    pub cache_creation_input_tokens: Option<u64>,
+}
+
+/// System-level message metadata.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SystemMessage {
+    /// System message type (e.g. `"turn_duration"`).
+    #[serde(rename = "type")]
+    pub msg_type: Option<String>,
+    /// Duration in milliseconds (for `turn_duration` entries).
+    pub duration_ms: Option<u64>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// An assistant message with `text` and `thinking` blocks deserializes correctly.
+    #[test]
+    fn parse_assistant_text_and_thinking() {
+        let json = r#"{
+            "type": "assistant",
+            "session_id": "abc123",
+            "message": {
+                "role": "assistant",
+                "content": [
+                    {"type": "thinking", "thinking": "Let me think about this."},
+                    {"type": "text", "text": "Here is my answer."}
+                ],
+                "usage": {
+                    "input_tokens": 100,
+                    "output_tokens": 50
+                },
+                "model": "claude-opus-4-5"
+            }
+        }"#;
+
+        let entry: RawEntry = serde_json::from_str(json).expect("should parse");
+        assert_eq!(entry.entry_type.as_deref(), Some("assistant"));
+        let msg = entry.message.expect("should have message");
+        assert_eq!(msg.role.as_deref(), Some("assistant"));
+        assert_eq!(msg.model.as_deref(), Some("claude-opus-4-5"));
+
+        let usage = msg.usage.expect("should have usage");
+        assert_eq!(usage.input_tokens, Some(100));
+        assert_eq!(usage.output_tokens, Some(50));
+
+        if let Some(MessageContent::Blocks(blocks)) = msg.content {
+            assert_eq!(blocks.len(), 2);
+            assert!(matches!(blocks[0], ContentBlock::Thinking { .. }));
+            assert!(matches!(blocks[1], ContentBlock::Text { .. }));
+        } else {
+            panic!("expected Blocks content");
+        }
+    }
+
+    /// A user message with a `tool_result` block deserializes correctly.
+    #[test]
+    fn parse_user_tool_result() {
+        let json = r#"{
+            "type": "user",
+            "session_id": "def456",
+            "message": {
+                "role": "user",
+                "content": [
+                    {
+                        "type": "tool_result",
+                        "tool_use_id": "toolu_01",
+                        "content": "command output here",
+                        "is_error": false
+                    }
+                ]
+            }
+        }"#;
+
+        let entry: RawEntry = serde_json::from_str(json).expect("should parse");
+        let msg = entry.message.expect("should have message");
+        if let Some(MessageContent::Blocks(blocks)) = msg.content {
+            assert_eq!(blocks.len(), 1);
+            if let ContentBlock::ToolResult {
+                tool_use_id,
+                is_error,
+                ..
+            } = &blocks[0]
+            {
+                assert_eq!(tool_use_id, "toolu_01");
+                assert_eq!(*is_error, Some(false));
+            } else {
+                panic!("expected ToolResult block");
+            }
+        } else {
+            panic!("expected Blocks content");
+        }
+    }
+
+    /// A `RawEntry` with unknown fields deserializes without error.
+    #[test]
+    fn raw_entry_unknown_fields_ok() {
+        let json = r#"{
+            "type": "progress",
+            "session_id": "ghi789",
+            "permissionMode": "auto",
+            "agentId": "some-agent-id",
+            "planContent": {"steps": []},
+            "unknownFutureField": true
+        }"#;
+
+        let entry: RawEntry =
+            serde_json::from_str(json).expect("should parse despite unknown fields");
+        assert_eq!(entry.entry_type.as_deref(), Some("progress"));
+        // Extra fields captured in the flatten map
+        assert!(entry.extra.contains_key("permissionMode"));
+        assert!(entry.extra.contains_key("agentId"));
+    }
+
+    /// A message with `content` as a plain string produces `MessageContent::Text`.
+    #[test]
+    fn parse_message_content_as_string() {
+        let json = r#"{
+            "type": "user",
+            "session_id": "jkl012",
+            "message": {
+                "role": "user",
+                "content": "Hello, Claude!"
+            }
+        }"#;
+
+        let entry: RawEntry = serde_json::from_str(json).expect("should parse");
+        let msg = entry.message.expect("should have message");
+        if let Some(MessageContent::Text(text)) = msg.content {
+            assert_eq!(text, "Hello, Claude!");
+        } else {
+            panic!("expected Text content");
+        }
+    }
+}
diff --git a/src/models/stats.rs b/src/models/stats.rs
new file mode 100644
index 0000000..9e80e5b
--- /dev/null
+++ b/src/models/stats.rs
@@ -0,0 +1,181 @@
+//! Session statistics computed from raw JSONL entries.
+
+use std::collections::HashMap;
+
+use crate::models::session::{ContentBlock, MessageContent, RawEntry};
+
+/// Aggregated statistics for a session.
+#[derive(Debug, Clone, Default)]
+pub struct SessionStats {
+    /// Total input tokens across all assistant messages.
+    pub input_tokens: u64,
+    /// Total output tokens across all assistant messages.
+    pub output_tokens: u64,
+    /// Total cache-read tokens across all assistant messages.
+    pub cache_read_tokens: u64,
+    /// Total cache-creation tokens across all assistant messages.
+    pub cache_creation_tokens: u64,
+    /// Tool call counts by tool name.
+    pub tool_calls: HashMap<String, u64>,
+    /// Total duration in milliseconds (from system `turn_duration` entries).
+    pub duration_ms: u64,
+    /// Model identifier from the first assistant message that has one.
+    pub model: Option<String>,
+    /// Total number of user messages.
+    pub user_message_count: u64,
+    /// Total number of assistant messages.
+    pub assistant_message_count: u64,
+}
+
+/// Compute session statistics from a slice of raw entries.
+pub fn compute_stats(entries: &[RawEntry]) -> SessionStats {
+    let mut stats = SessionStats::default();
+    for entry in entries {
+        match entry.entry_type.as_deref() {
+            Some("assistant") => {
+                if let Some(msg) = &entry.message {
+                    stats.assistant_message_count += 1;
+                    // Accumulate usage tokens.
+                    if let Some(usage) = &msg.usage {
+                        stats.input_tokens += usage.input_tokens.unwrap_or(0);
+                        stats.output_tokens += usage.output_tokens.unwrap_or(0);
+                        stats.cache_read_tokens += usage.cache_read_input_tokens.unwrap_or(0);
+                        stats.cache_creation_tokens +=
+                            usage.cache_creation_input_tokens.unwrap_or(0);
+                    }
+                    // Capture model from first assistant message that has one.
+                    if stats.model.is_none() {
+                        stats.model = msg.model.clone();
+                    }
+                    // Count tool_use blocks.
+                    if let Some(MessageContent::Blocks(blocks)) = &msg.content {
+                        for block in blocks {
+                            if let ContentBlock::ToolUse { name, .. } = block {
+                                *stats.tool_calls.entry(name.clone()).or_insert(0) += 1;
+                            }
+                        }
+                    }
+                }
+            }
+            Some("user") => {
+                if entry.message.is_some() {
+                    stats.user_message_count += 1;
+                }
+            }
+            Some("system") => {
+                // Accumulate duration from `turn_duration` system entries.
+                if let Some(dur) = entry.duration_ms {
+                    stats.duration_ms += dur;
+                }
+            }
+            _ => {}
+        }
+    }
+    stats
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::session::{Message, MessageContent, RawEntry, Usage};
+
+    fn make_entry(entry_type: &str) -> RawEntry {
+        RawEntry {
+            entry_type: Some(entry_type.to_string()),
+            session_id: None,
+            parent_session_id: None,
+            message: None,
+            system_message: None,
+            cwd: None,
+            timestamp: None,
+            duration_ms: None,
+            extra: Default::default(),
+        }
+    }
+
+    /// Empty entry slice produces all-zero stats with no model.
+    #[test]
+    fn empty_entries_all_zeros() {
+        let stats = compute_stats(&[]);
+        assert_eq!(stats.input_tokens, 0);
+        assert_eq!(stats.output_tokens, 0);
+        assert_eq!(stats.cache_read_tokens, 0);
+        assert_eq!(stats.cache_creation_tokens, 0);
+        assert_eq!(stats.duration_ms, 0);
+        assert_eq!(stats.user_message_count, 0);
+        assert_eq!(stats.assistant_message_count, 0);
+        assert!(stats.model.is_none());
+        assert!(stats.tool_calls.is_empty());
+    }
+
+    /// A single assistant entry with usage produces correct token counts.
+    #[test]
+    fn single_assistant_usage() {
+        let mut entry = make_entry("assistant");
+        entry.message = Some(Message {
+            role: Some("assistant".to_string()),
+            content: None,
+            usage: Some(Usage {
+                input_tokens: Some(200),
+                output_tokens: Some(80),
+                cache_read_input_tokens: Some(50),
+                cache_creation_input_tokens: Some(10),
+            }),
+            model: Some("claude-opus-4-5".to_string()),
+            stop_reason: None,
+        });
+
+        let stats = compute_stats(&[entry]);
+        assert_eq!(stats.input_tokens, 200);
+        assert_eq!(stats.output_tokens, 80);
+        assert_eq!(stats.cache_read_tokens, 50);
+        assert_eq!(stats.cache_creation_tokens, 10);
+        assert_eq!(stats.assistant_message_count, 1);
+        assert_eq!(stats.model.as_deref(), Some("claude-opus-4-5"));
+    }
+
+    /// An assistant entry with tool_use blocks increments tool call counts.
+    #[test]
+    fn assistant_tool_use_counts() {
+        let mut entry = make_entry("assistant");
+        entry.message = Some(Message {
+            role: Some("assistant".to_string()),
+            content: Some(MessageContent::Blocks(vec![
+                ContentBlock::ToolUse {
+                    id: "t1".to_string(),
+                    name: "Bash".to_string(),
+                    input: serde_json::Value::Null,
+                },
+                ContentBlock::ToolUse {
+                    id: "t2".to_string(),
+                    name: "Read".to_string(),
+                    input: serde_json::Value::Null,
+                },
+                ContentBlock::ToolUse {
+                    id: "t3".to_string(),
+                    name: "Bash".to_string(),
+                    input: serde_json::Value::Null,
+                },
+            ])),
+            usage: None,
+            model: None,
+            stop_reason: None,
+        });
+
+        let stats = compute_stats(&[entry]);
+        assert_eq!(stats.tool_calls.get("Bash"), Some(&2));
+        assert_eq!(stats.tool_calls.get("Read"), Some(&1));
+    }
+
+    /// Multiple entries with `duration_ms` are summed correctly.
+    #[test]
+    fn duration_summed_across_entries() {
+        let mut e1 = make_entry("system");
+        e1.duration_ms = Some(1000);
+        let mut e2 = make_entry("system");
+        e2.duration_ms = Some(2500);
+
+        let stats = compute_stats(&[e1, e2]);
+        assert_eq!(stats.duration_ms, 3500);
+    }
+}