diff --git a/server/src/chat/commands/diff.rs b/server/src/chat/commands/diff.rs
new file mode 100644
index 00000000..751393ca
--- /dev/null
+++ b/server/src/chat/commands/diff.rs
@@ -0,0 +1,259 @@
+//! Handler for the `diff` command.
+//!
+//! Shows the git diff from the configured main branch to the story's worktree
+//! HEAD, formatted for readability in chat.
+
+use super::CommandContext;
+use std::path::Path;
+use std::process::Command;
+
+/// Display the git diff from the configured main branch to a story's worktree HEAD.
+///
+/// Usage: `diff <number>`
+pub(super) fn handle_diff(ctx: &CommandContext) -> Option<String> {
+    let num_str = ctx.args.trim();
+    if num_str.is_empty() {
+        return Some(format!(
+            "Usage: `{} diff <number>`\n\nShows the git diff from the main branch to the story's worktree HEAD.",
+            ctx.bot_name
+        ));
+    }
+    if !num_str.chars().all(|c| c.is_ascii_digit()) {
+        return Some(format!(
+            "Invalid story number: `{num_str}`. Usage: `{} diff <number>`",
+            ctx.bot_name
+        ));
+    }
+
+    let story_id = match find_story_id(num_str) {
+        Some(id) => id,
+        None => {
+            return Some(format!(
+                "No story with number **{num_str}** found in the pipeline."
+            ));
+        }
+    };
+
+    let wt_path = crate::worktree::worktree_path(ctx.project_root, &story_id);
+    if !wt_path.is_dir() {
+        return Some(format!(
+            "Story **{num_str}** has no worktree. The diff is only available once a coder has started working on it."
+        ));
+    }
+
+    let base_branch = resolve_base_branch(ctx.project_root);
+    let range = format!("{base_branch}...HEAD");
+
+    let stat = run_git(&wt_path, &["diff", "--stat", &range]);
+    let diff = run_git(&wt_path, &["diff", &range]);
+
+    let mut out = format!("## Diff — story {num_str} vs `{base_branch}`\n\n");
+
+    if stat.is_empty() && diff.is_empty() {
+        out.push_str("*(no changes relative to main branch)*\n");
+        return Some(out);
+    }
+
+    if !stat.is_empty() {
+        out.push_str("**Changed files:**\n```\n");
+        out.push_str(&stat);
+        out.push_str("\n```\n\n");
+    }
+
+    if !diff.is_empty() {
+        const MAX_DIFF_BYTES: usize = 8_000;
+        if diff.len() > MAX_DIFF_BYTES {
+            let truncated = truncate_at_char_boundary(&diff, MAX_DIFF_BYTES);
+            out.push_str("**Diff** *(truncated — showing first 8 KB)*:\n```diff\n");
+            out.push_str(truncated);
+            out.push_str("\n... (truncated)\n```\n");
+        } else {
+            out.push_str("**Diff:**\n```diff\n");
+            out.push_str(&diff);
+            out.push_str("\n```\n");
+        }
+    }
+
+    Some(out)
+}
+
+/// Find the story_id in the pipeline whose numeric prefix matches `num_str`.
+fn find_story_id(num_str: &str) -> Option<String> {
+    let items = crate::pipeline_state::read_all_typed();
+    items.into_iter().find_map(|item| {
+        let file_num = item
+            .story_id
+            .0
+            .split('_')
+            .next()
+            .filter(|s| !s.is_empty() && s.chars().all(|c| c.is_ascii_digit()))
+            .unwrap_or("");
+        if file_num == num_str {
+            Some(item.story_id.0.clone())
+        } else {
+            None
+        }
+    })
+}
+
+/// Return the configured base branch, or auto-detect it from the project root HEAD.
+fn resolve_base_branch(project_root: &Path) -> String {
+    crate::config::ProjectConfig::load(project_root)
+        .ok()
+        .and_then(|c| c.base_branch)
+        .unwrap_or_else(|| {
+            Command::new("git")
+                .args(["rev-parse", "--abbrev-ref", "HEAD"])
+                .current_dir(project_root)
+                .output()
+                .ok()
+                .filter(|o| o.status.success())
+                .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+                .unwrap_or_else(|| "master".to_string())
+        })
+}
+
+/// Run a git command in `dir`, returning trimmed stdout (empty string on failure).
+fn run_git(dir: &Path, args: &[&str]) -> String {
+    Command::new("git")
+        .args(args)
+        .current_dir(dir)
+        .output()
+        .ok()
+        .filter(|o| o.status.success())
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .unwrap_or_default()
+}
+
+/// Truncate `s` to at most `max_bytes` bytes without splitting a UTF-8 character.
+fn truncate_at_char_boundary(s: &str, max_bytes: usize) -> &str {
+    if s.len() <= max_bytes {
+        return s;
+    }
+    let mut boundary = max_bytes;
+    while !s.is_char_boundary(boundary) {
+        boundary -= 1;
+    }
+    &s[..boundary]
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::agents::AgentPool;
+    use std::collections::HashSet;
+    use std::sync::{Arc, Mutex};
+
+    use super::super::{CommandDispatch, try_handle_command};
+
+    fn diff_cmd(root: &std::path::Path, args: &str) -> Option<String> {
+        let agents = Arc::new(AgentPool::new_test(3000));
+        let ambient_rooms = Arc::new(Mutex::new(HashSet::new()));
+        let room_id = "!test:example.com".to_string();
+        let dispatch = CommandDispatch {
+            bot_name: "Timmy",
+            bot_user_id: "@timmy:homeserver.local",
+            project_root: root,
+            agents: &agents,
+            ambient_rooms: &ambient_rooms,
+            room_id: &room_id,
+        };
+        try_handle_command(&dispatch, &format!("@timmy diff {args}"))
+    }
+
+    #[test]
+    fn diff_command_is_registered() {
+        let found = super::super::commands().iter().any(|c| c.name == "diff");
+        assert!(found, "diff command must be in the registry");
+    }
+
+    #[test]
+    fn diff_command_appears_in_help() {
+        let result = super::super::tests::try_cmd_addressed(
+            "Timmy",
+            "@timmy:homeserver.local",
+            "@timmy help",
+        );
+        let output = result.unwrap();
+        assert!(
+            output.contains("diff"),
+            "help should list diff command: {output}"
+        );
+    }
+
+    #[test]
+    fn diff_command_no_args_returns_usage() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let output = diff_cmd(tmp.path(), "").unwrap();
+        assert!(
+            output.contains("Usage"),
+            "no args should show usage: {output}"
+        );
+    }
+
+    #[test]
+    fn diff_command_non_numeric_returns_error() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let output = diff_cmd(tmp.path(), "abc").unwrap();
+        assert!(
+            output.contains("Invalid"),
+            "non-numeric arg should return error: {output}"
+        );
+    }
+
+    #[test]
+    fn diff_command_story_not_found_returns_friendly_message() {
+        crate::db::ensure_content_store();
+        let tmp = tempfile::TempDir::new().unwrap();
+        let output = diff_cmd(tmp.path(), "99993").unwrap();
+        assert!(
+            output.contains("99993"),
+            "message should include story number: {output}"
+        );
+        assert!(
+            output.contains("found") || output.contains("pipeline"),
+            "message should explain not found: {output}"
+        );
+    }
+
+    #[test]
+    fn diff_command_no_worktree_returns_clear_error() {
+        use crate::chat::test_helpers::write_story_file;
+        let tmp = tempfile::TempDir::new().unwrap();
+        write_story_file(
+            tmp.path(),
+            "2_current",
+            "55551_story_no_worktree.md",
+            "---\nname: No Worktree\n---\n",
+        );
+        let output = diff_cmd(tmp.path(), "55551").unwrap();
+        assert!(
+            output.contains("worktree")
+                || output.contains("no worktree")
+                || output.contains("Worktree"),
+            "should report missing worktree: {output}"
+        );
+    }
+
+    #[test]
+    fn truncate_at_char_boundary_short_string() {
+        let s = "hello";
+        assert_eq!(truncate_at_char_boundary(s, 100), "hello");
+    }
+
+    #[test]
+    fn truncate_at_char_boundary_exact_limit() {
+        let s = "hello";
+        assert_eq!(truncate_at_char_boundary(s, 5), "hello");
+    }
+
+    #[test]
+    fn truncate_at_char_boundary_over_limit() {
+        let s = "hello world";
+        assert_eq!(truncate_at_char_boundary(s, 5), "hello");
+    }
+}
diff --git a/server/src/chat/commands/mod.rs b/server/src/chat/commands/mod.rs
index ddf23766..81311094 100644
--- a/server/src/chat/commands/mod.rs
+++ b/server/src/chat/commands/mod.rs
@@ -11,6 +11,7 @@ mod backlog;
 mod cost;
 mod coverage;
 mod depends;
+mod diff;
 mod freeze;
 mod git;
 mod help;
@@ -164,6 +165,11 @@ pub fn commands() -> &'static [BotCommand] {
             description: "Display the full text of a work item: `show <number>`",
             handler: show::handle_show,
         },
+        BotCommand {
+            name: "diff",
+            description: "Show git diff from main branch to story worktree HEAD: `diff <number>`",
+            handler: diff::handle_diff,
+        },
         BotCommand {
             name: "overview",
             description: "Show implementation summary for a merged story: `overview <number>`",