diff --git a/rust/fleet-data/src/git.rs b/rust/fleet-data/src/git.rs
new file mode 100644
index 0000000..bf56bff
--- /dev/null
+++ b/rust/fleet-data/src/git.rs
@@ -0,0 +1,285 @@
+//! Cross-lane PR awareness via `gh pr list` + `git merge-base`.
+//!
+//! The fleet's `task_ready_for_agent` queue treats every candidate task as
+//! independent. In practice, lane A may produce a file that lane B's task
+//! depends on — and when A's PR isn't yet merged into B's base, B will pick
+//! up the work and immediately rediscover the dependency at PR time. This
+//! module is the data-layer half of the fix: it answers "which open PRs
+//! touch which files, and is any given branch already past PR N's head."
+//!
+//! The colony-side consumer (`readyScopeOverlapWarnings` in
+//! `colony/ready-queue.ts`) emits a `merge_pending` warning in `next_action`
+//! when a candidate's file scope intersects an open PR's fileset that the
+//! candidate's base has not yet absorbed. Per `task_claim_file`'s docstring,
+//! Colony's coordination is "soft — never blocks writes" — so this surfaces
+//! as a warning that the autopilot deprioritizes, not a hard block.
+//!
+//! ## Failure posture
+//!
+//! Mirrors [`crate::accounts`]: a missing `gh` binary, an unauthenticated
+//! CLI, or a network failure all return `Ok(vec![])` from
+//! [`open_prs_with_files`]. A dashboard without `gh` access should render
+//! no merge-pending warnings, not crash. For [`branch_contains_pr`], a
+//! lookup failure conservatively returns `Ok(false)` — "I cannot prove
+//! this PR is merged" is the safe default that produces a warning.
+
+use serde::{Deserialize, Serialize};
+
+/// One open PR's metadata + fileset, as far as the ready-queue cares.
+///
+/// The fileset is *paths only* — `additions` / `deletions` counts from
+/// `gh pr list --json files` are dropped on parse, because cross-lane
+/// prereq detection only needs the set of paths.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PrFileset {
+    pub number: u64,
+    pub head_ref: String,
+    pub base_ref: String,
+    pub files: Vec<String>,
+}
+
+impl PrFileset {
+    /// `true` if any of this PR's files appears in the candidate scope.
+    /// Both sides are matched exact-path; directory globbing is the
+    /// caller's job (the colony side already normalizes to file paths).
+    pub fn overlaps(&self, scope: &[String]) -> bool {
+        self.files.iter().any(|f| scope.iter().any(|s| s == f))
+    }
+}
+
+/// Raw `gh pr list --json` shape — `files` is an array of `{path, additions, …}`.
+#[derive(Deserialize)]
+struct RawPr {
+    number: u64,
+    #[serde(rename = "headRefName")]
+    head_ref: String,
+    #[serde(rename = "baseRefName")]
+    base_ref: String,
+    files: Vec<RawFile>,
+}
+
+#[derive(Deserialize)]
+struct RawFile {
+    path: String,
+}
+
+/// Parse `gh pr list --json number,headRefName,baseRefName,files` stdout
+/// into typed [`PrFileset`] rows. Pure — feed it captured JSON in tests.
+///
+/// A malformed payload returns an empty Vec rather than an error; same
+/// posture as [`crate::accounts::parse`] — the dashboard's "no information"
+/// path is an empty fleet, not a panic.
+pub fn parse(stdout: &str) -> Vec<PrFileset> {
+    let raws: Vec<RawPr> = serde_json::from_str(stdout).unwrap_or_default();
+    raws.into_iter()
+        .map(|r| PrFileset {
+            number: r.number,
+            head_ref: r.head_ref,
+            base_ref: r.base_ref,
+            files: r.files.into_iter().map(|f| f.path).collect(),
+        })
+        .collect()
+}
+
+/// `true` when `branch` is at or past `pr_head` — i.e. the PR's head commit
+/// is already an ancestor of `branch`. Wraps
+/// `git merge-base --is-ancestor <pr_head> <branch>` (exit 0 = ancestor).
+///
+/// Returns `Ok(false)` on any failure path (unknown ref, no git on PATH) —
+/// for cross-lane warnings, "I cannot prove this PR is merged" is the safe
+/// default that produces a warning rather than silently suppressing it.
+pub fn branch_contains_pr(branch: &str, pr_head: &str) -> std::io::Result<bool> {
+    let status = std::process::Command::new("git")
+        .args(["merge-base", "--is-ancestor", pr_head, branch])
+        .stdout(std::process::Stdio::null())
+        .stderr(std::process::Stdio::null())
+        .status()?;
+    Ok(status.success())
+}
+
+/// Shell out to `gh pr list` and parse the result.
+///
+/// Returns `Ok(vec![])` when `gh` is missing, unauthenticated, or returns a
+/// non-zero exit — a dashboard without `gh` access renders no warnings, not
+/// a crash. `--limit 100` is enough for any plausible fleet; a 100-PR
+/// backlog is itself a signal worth surfacing elsewhere.
+pub fn open_prs_with_files() -> std::io::Result<Vec<PrFileset>> {
+    let out = match std::process::Command::new("gh")
+        .args([
+            "pr",
+            "list",
+            "--state",
+            "open",
+            "--json",
+            "number,headRefName,baseRefName,files",
+            "--limit",
+            "100",
+        ])
+        .output()
+    {
+        Ok(o) => o,
+        Err(_) => return Ok(Vec::new()),
+    };
+    if !out.status.success() {
+        return Ok(Vec::new());
+    }
+    Ok(parse(&String::from_utf8_lossy(&out.stdout)))
+}
+
+fn cache() -> &'static crate::cache::TtlCache<Vec<PrFileset>> {
+    static CACHE: std::sync::OnceLock<crate::cache::TtlCache<Vec<PrFileset>>> =
+        std::sync::OnceLock::new();
+    // 45s TTL: `gh pr list` is rate-limited (5000/hr authenticated).
+    // Four dashboard binaries each refreshing once per TTL window peak at
+    // ~320 calls/hr — two orders of magnitude under the cap, while still
+    // surfacing a freshly-opened PR within ~one tick of a minute.
+    CACHE.get_or_init(|| crate::cache::TtlCache::new(std::time::Duration::from_secs(45)))
+}
+
+/// Cached variant of [`open_prs_with_files`]. The fleet's dashboard tick
+/// is 250 ms; calling the raw fn every tick would burn ~14k `gh`
+/// invocations per hour per binary and trip GitHub's rate limit. The 45 s
+/// TTL is tuned so a freshly-opened PR shows up within ~one minute and
+/// readers stay cheap.
+pub fn load_live_cached() -> std::io::Result<Vec<PrFileset>> {
+    cache().get_or_refresh(open_prs_with_files)
+}
+
+/// Drop the cached PR list so the next [`load_live_cached`] re-shells.
+/// Useful after observing a `gh pr merge` / `gh pr close` locally, so the
+/// dashboard's warnings clear immediately rather than after the TTL.
+pub fn invalidate_cache() {
+    cache().invalidate();
+}
+
+/// Find every open PR whose fileset intersects `scope` and that has not yet
+/// been merged into `branch`'s history. This is the question
+/// `readyScopeOverlapWarnings` actually asks: "if lane B claims these
+/// files, what un-merged PRs already touch them and would conflict on
+/// rebase?"
+///
+/// Returns `Ok(vec![])` when nothing matches, including the trivial "no
+/// open PRs" / "no `gh` on PATH" cases. The caller (colony) folds each
+/// returned PR into a `merge_pending` warning on the task's `next_action`.
+pub fn merge_pending_overlap(
+    branch: &str,
+    scope: &[String],
+) -> std::io::Result<Vec<PrFileset>> {
+    let prs = load_live_cached()?;
+    let mut out = Vec::new();
+    for pr in prs {
+        if !pr.overlaps(scope) {
+            continue;
+        }
+        // Skip PRs whose head is already an ancestor of `branch` — they're
+        // either merged-and-pulled or this lane was cut from a tip that
+        // already contained them. Either way, no merge-pending conflict.
+        if branch_contains_pr(branch, &pr.head_ref).unwrap_or(false) {
+            continue;
+        }
+        out.push(pr);
+    }
+    Ok(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const FIXTURE: &str = r#"[
+  {
+    "number": 24,
+    "headRefName": "agent/claude/fleet-state-live-data-2026-05-14-20-19",
+    "baseRefName": "main",
+    "files": [
+      {"path": "rust/fleet-state/src/main.rs", "additions": 247, "deletions": 40},
+      {"path": "rust/fleet-data/src/fleet.rs", "additions": 419, "deletions": 0}
+    ]
+  },
+  {
+    "number": 25,
+    "headRefName": "agent/codex/overlays-phase5-sub-1-spotlight",
+    "baseRefName": "main",
+    "files": [
+      {"path": "rust/fleet-ui/src/overlay/spotlight.rs", "additions": 312, "deletions": 0}
+    ]
+  }
+]"#;
+
+    #[test]
+    fn parses_pr_list_json() {
+        let prs = parse(FIXTURE);
+        assert_eq!(prs.len(), 2);
+        assert_eq!(prs[0].number, 24);
+        assert_eq!(
+            prs[0].head_ref,
+            "agent/claude/fleet-state-live-data-2026-05-14-20-19"
+        );
+        assert_eq!(prs[0].base_ref, "main");
+        assert_eq!(
+            prs[0].files,
+            vec![
+                "rust/fleet-state/src/main.rs".to_string(),
+                "rust/fleet-data/src/fleet.rs".to_string(),
+            ]
+        );
+        assert_eq!(prs[1].number, 25);
+        assert_eq!(prs[1].files.len(), 1);
+    }
+
+    #[test]
+    fn malformed_json_yields_empty_vec() {
+        assert!(parse("not json at all").is_empty());
+        assert!(parse("").is_empty());
+        assert!(parse("{}").is_empty(), "object, not array");
+    }
+
+    #[test]
+    fn missing_files_field_drops_the_row() {
+        // `files` is required — a row without it fails the whole array parse,
+        // which unwrap_or_default turns into an empty Vec. That's intentional:
+        // a partial gh response shouldn't half-populate the warnings list.
+        let prs =
+            parse(r#"[{"number": 1, "headRefName": "x", "baseRefName": "main"}]"#);
+        assert!(prs.is_empty());
+    }
+
+    fn pr(number: u64, files: &[&str]) -> PrFileset {
+        PrFileset {
+            number,
+            head_ref: format!("agent/x/{number}"),
+            base_ref: "main".into(),
+            files: files.iter().map(|s| s.to_string()).collect(),
+        }
+    }
+
+    #[test]
+    fn overlap_matches_exact_path() {
+        let p = pr(
+            1,
+            &["rust/fleet-data/src/fleet.rs", "rust/fleet-data/src/tmux.rs"],
+        );
+        assert!(p.overlaps(&["rust/fleet-data/src/fleet.rs".to_string()]));
+        assert!(p.overlaps(&[
+            "unrelated.txt".to_string(),
+            "rust/fleet-data/src/tmux.rs".to_string()
+        ]));
+        assert!(!p.overlaps(&["rust/fleet-state/src/main.rs".to_string()]));
+        assert!(!p.overlaps(&[]));
+    }
+
+    #[test]
+    fn empty_pr_files_never_overlaps() {
+        let p = pr(2, &[]);
+        assert!(!p.overlaps(&["any.rs".to_string()]));
+    }
+
+    #[test]
+    fn overlap_is_directional() {
+        // Path strings must match exactly — `fleet-data/src/fleet.rs` doesn't
+        // overlap `rust/fleet-data/src/fleet.rs`. Directory-prefix matching
+        // is the caller's job; the colony side normalizes both before calling.
+        let p = pr(3, &["rust/fleet-data/src/fleet.rs"]);
+        assert!(!p.overlaps(&["fleet-data/src/fleet.rs".to_string()]));
+    }
+}
diff --git a/rust/fleet-data/src/lib.rs b/rust/fleet-data/src/lib.rs
index 91eaab1..fae3018 100644
--- a/rust/fleet-data/src/lib.rs
+++ b/rust/fleet-data/src/lib.rs
@@ -3,6 +3,7 @@
 pub mod accounts;
 pub mod cache;
 pub mod fleet;
+pub mod git;
 pub mod panes;
 pub mod plan;
 pub mod tmux;