diff --git a/CHANGELOG.md b/CHANGELOG.md
index c6b312cd..df91d24d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,16 @@
 
 ## [Unreleased]
 
+### Added
+
+- **`rivet coverage --aggregate <FILE>...`** (#188 sub-issue 3). File-based
+  cross-repo V&V matrix aggregator: each repo's CI emits its
+  `rivet coverage --matrix --format json`, a top-level job merges them
+  into one matrix (text / markdown / html / json). Needs no GitHub API
+  access — inputs are plain files; duplicate `(repo, id)` rows are
+  coalesced so re-runs are idempotent, and the merged JSON re-feeds the
+  aggregator unchanged.
+
 ## [0.9.0] — 2026-05-11
 
 Theme: backlog drain. Ships the rivet-bundle command, the s-expr
diff --git a/rivet-cli/src/main.rs b/rivet-cli/src/main.rs
index 898f93d7..e01cf124 100644
--- a/rivet-cli/src/main.rs
+++ b/rivet-cli/src/main.rs
@@ -423,6 +423,15 @@ enum Command {
         /// markdown, or html output. Mutually exclusive with `--tests`.
         #[arg(long, conflicts_with = "tests")]
         matrix: bool,
+
+        /// Aggregate one or more JSON matrices produced by
+        /// `rivet coverage --matrix --format json` (rivet#188 sub-issue 3,
+        /// the file-based cross-repo aggregator). Each CI job emits its
+        /// repo's JSON; a top-level job merges them with this flag. Implies
+        /// `--matrix`; the local project is not read. Repeat the flag or
+        /// pass several paths.
+        #[arg(long = "aggregate", value_name = "FILE", num_args = 1.., conflicts_with = "tests")]
+        aggregate: Vec<PathBuf>,
     },
 
     /// Generate a traceability matrix
@@ -1732,8 +1741,11 @@ fn run(cli: Cli) -> Result<bool> {
             scan_paths,
             baseline,
             matrix,
+            aggregate,
         } => {
-            if *matrix {
+            if !aggregate.is_empty() {
+                cmd_coverage_matrix_aggregate(format, aggregate)
+            } else if *matrix {
                 cmd_coverage_matrix(&cli, format, baseline.as_deref())
             } else if *tests {
                 cmd_coverage_tests(&cli, format, scan_paths)
@@ -5827,6 +5839,104 @@ fn cmd_coverage_matrix(cli: &Cli, format: &str, baseline_name: Option<&str>) ->
     Ok(true)
 }
 
+// ── V&V coverage matrix — cross-repo aggregator (rivet#188 sub-issue 3) ─
+//
+// The file-based aggregator: each repo's CI runs
+// `rivet coverage --matrix --format json` and uploads the result; a
+// top-level job collects those JSON files and merges them with
+// `rivet coverage --aggregate a.json b.json ... --format {text,markdown,html,json}`.
+// No GitHub API access is needed — the inputs are plain files.
+
+fn json_string_list(value: Option<&serde_json::Value>) -> Vec<String> {
+    value
+        .and_then(serde_json::Value::as_array)
+        .map(|seq| {
+            seq.iter()
+                .filter_map(|v| v.as_str())
+                .map(str::to_owned)
+                .collect()
+        })
+        .unwrap_or_default()
+}
+
+/// Parse one JSON matrix file (the envelope emitted by `render_matrix_json`)
+/// into `RepoStatusRow`s.
+fn parse_matrix_json_file(path: &std::path::Path) -> Result<Vec<RepoStatusRow>> {
+    let text = std::fs::read_to_string(path)
+        .with_context(|| format!("reading matrix JSON {}", path.display()))?;
+    let value: serde_json::Value = serde_json::from_str(&text)
+        .with_context(|| format!("parsing matrix JSON {}", path.display()))?;
+    let repos = value
+        .get("repos")
+        .and_then(serde_json::Value::as_array)
+        .ok_or_else(|| {
+            anyhow::anyhow!(
+                "{}: not a `rivet coverage --matrix --format json` document \
+                 (missing `repos` array)",
+                path.display()
+            )
+        })?;
+    let mut rows = Vec::with_capacity(repos.len());
+    for repo in repos {
+        let repo_name = repo.get("repo").and_then(serde_json::Value::as_str);
+        let id = repo.get("id").and_then(serde_json::Value::as_str);
+        let (id, repo_name) = match (id, repo_name) {
+            (Some(i), Some(r)) => (i.to_owned(), r.to_owned()),
+            (Some(i), None) => (i.to_owned(), i.to_owned()),
+            (None, Some(r)) => (r.to_owned(), r.to_owned()),
+            (None, None) => {
+                anyhow::bail!(
+                    "{}: a `repos` entry has neither `id` nor `repo`",
+                    path.display()
+                )
+            }
+        };
+        let notes = repo
+            .get("notes")
+            .and_then(serde_json::Value::as_str)
+            .map(str::trim)
+            .filter(|s| !s.is_empty())
+            .map(str::to_owned);
+        rows.push(RepoStatusRow {
+            id,
+            repo: repo_name,
+            applied: json_string_list(repo.get("techniques_applied")),
+            gated: json_string_list(repo.get("techniques_gated_in_ci")),
+            notes,
+        });
+    }
+    Ok(rows)
+}
+
+/// Merge several JSON matrices into one and render it.
+fn cmd_coverage_matrix_aggregate(format: &str, paths: &[PathBuf]) -> Result<bool> {
+    validate_format(format, &["text", "json", "markdown", "html"])?;
+
+    let mut rows: Vec<RepoStatusRow> = Vec::new();
+    let mut seen: std::collections::HashSet<(String, String)> = std::collections::HashSet::new();
+    for path in paths {
+        for row in parse_matrix_json_file(path)? {
+            // First occurrence of a (repo, id) pair wins; later files that
+            // re-state the same row are ignored so re-running the aggregator
+            // over overlapping inputs is idempotent.
+            if seen.insert((row.repo.clone(), row.id.clone())) {
+                rows.push(row);
+            }
+        }
+    }
+    rows.sort_by(|a, b| a.repo.cmp(&b.repo).then_with(|| a.id.cmp(&b.id)));
+    let cols = matrix_columns(&rows);
+
+    match format {
+        "json" => render_matrix_json(&rows, &cols),
+        "markdown" => render_matrix_markdown(&rows, &cols),
+        "html" => render_matrix_html(&rows, &cols),
+        _ => render_matrix_text(&rows, &cols),
+    }
+
+    Ok(true)
+}
+
 /// Generate a traceability matrix.
 fn cmd_matrix(
     cli: &Cli,
diff --git a/rivet-cli/tests/cli_commands.rs b/rivet-cli/tests/cli_commands.rs
index a5120cc0..30cc9146 100644
--- a/rivet-cli/tests/cli_commands.rs
+++ b/rivet-cli/tests/cli_commands.rs
@@ -1948,6 +1948,180 @@ fn coverage_matrix_empty_project() {
     );
 }
 
+// ── rivet coverage --aggregate (rivet#188 sub-issue 3) ─────────────────
+
+/// Write two single-repo matrix JSON files (the shape
+/// `rivet coverage --matrix --format json` emits) into a fresh tmpdir and
+/// return `(tmpdir, path_a, path_b)`.
+fn aggregate_inputs() -> (tempfile::TempDir, std::path::PathBuf, std::path::PathBuf) {
+    let tmp = tempfile::tempdir().expect("create temp dir");
+    let a = tmp.path().join("rivet.json");
+    let b = tmp.path().join("loom.json");
+    std::fs::write(
+        &a,
+        r#"{"command":"coverage-matrix","columns":["kani","proptest"],
+            "repos":[{"id":"RS-RIVET","repo":"pulseengine/rivet",
+            "techniques_applied":["kani","proptest"],
+            "techniques_gated_in_ci":["proptest"],"notes":"ref",
+            "cells":[{"technique":"kani","status":"applied"},
+            {"technique":"proptest","status":"gated"}]}]}"#,
+    )
+    .expect("write a.json");
+    std::fs::write(
+        &b,
+        r#"{"command":"coverage-matrix","columns":["miri"],
+            "repos":[{"id":"RS-LOOM","repo":"pulseengine/loom",
+            "techniques_applied":["miri","kani"],"techniques_gated_in_ci":[],
+            "notes":null,"cells":[{"technique":"miri","status":"applied"}]}]}"#,
+    )
+    .expect("write b.json");
+    (tmp, a, b)
+}
+
+/// `--aggregate a.json b.json --format markdown` merges both repos into one
+/// table whose columns are the union of every input's techniques.
+#[test]
+fn coverage_aggregate_markdown_merges_repos() {
+    let (tmp, a, b) = aggregate_inputs();
+    let out = Command::new(rivet_bin())
+        .args([
+            "coverage",
+            "--aggregate",
+            a.to_str().unwrap(),
+            b.to_str().unwrap(),
+            "--format",
+            "markdown",
+        ])
+        .output()
+        .expect("coverage --aggregate --format markdown");
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        out.status.success(),
+        "must exit 0. stdout:\n{stdout}\nstderr:\n{stderr}"
+    );
+    assert!(
+        stdout.contains("# V&V coverage matrix"),
+        "heading. {stdout}"
+    );
+    for repo in ["pulseengine/rivet", "pulseengine/loom"] {
+        assert!(
+            stdout.contains(&format!("| {} |", repo)),
+            "row for {repo}. {stdout}"
+        );
+    }
+    // Union of columns across both files, sorted.
+    let header = stdout
+        .lines()
+        .find(|l| l.starts_with("| repo |"))
+        .expect("header row");
+    assert_eq!(
+        header, "| repo | kani | miri | proptest |",
+        "merged columns"
+    );
+    // rivet has proptest CI-gated.
+    assert!(stdout.contains('●'), "gated glyph somewhere. {stdout}");
+    drop(tmp);
+}
+
+/// The aggregate JSON output uses the same envelope as the per-repo
+/// command, so it can be fed straight back into `--aggregate`; duplicate
+/// (repo, id) rows are coalesced so re-runs are idempotent.
+#[test]
+fn coverage_aggregate_json_roundtrips_and_dedups() {
+    let (tmp, a, b) = aggregate_inputs();
+    // Pass `a` twice plus `b`; the duplicate must not produce a second row.
+    let out = Command::new(rivet_bin())
+        .args([
+            "coverage",
+            "--aggregate",
+            a.to_str().unwrap(),
+            a.to_str().unwrap(),
+            b.to_str().unwrap(),
+            "--format",
+            "json",
+        ])
+        .output()
+        .expect("coverage --aggregate --format json");
+    assert!(
+        out.status.success(),
+        "exit 0: {}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    let parsed: serde_json::Value = serde_json::from_str(&stdout).expect("aggregate JSON parses");
+    assert_eq!(
+        parsed.get("command").and_then(|v| v.as_str()),
+        Some("coverage-matrix")
+    );
+    let repos = parsed
+        .get("repos")
+        .and_then(|v| v.as_array())
+        .expect("repos array");
+    assert_eq!(repos.len(), 2, "duplicate row coalesced. {stdout}");
+    assert_eq!(
+        parsed
+            .get("columns")
+            .and_then(|v| v.as_array())
+            .map(Vec::len),
+        Some(3),
+        "merged column count. {stdout}"
+    );
+
+    // Re-feed the merged output back into the aggregator: same result.
+    let merged = tmp.path().join("merged.json");
+    std::fs::write(&merged, stdout.as_bytes()).expect("write merged.json");
+    let out2 = Command::new(rivet_bin())
+        .args([
+            "coverage",
+            "--aggregate",
+            merged.to_str().unwrap(),
+            "--format",
+            "json",
+        ])
+        .output()
+        .expect("re-aggregate");
+    assert!(out2.status.success(), "re-aggregate exit 0");
+    let reparsed: serde_json::Value =
+        serde_json::from_str(&String::from_utf8_lossy(&out2.stdout)).expect("re-parse");
+    assert_eq!(
+        reparsed
+            .get("repos")
+            .and_then(|v| v.as_array())
+            .map(Vec::len),
+        Some(2),
+        "round-trip preserves rows"
+    );
+}
+
+/// A non-JSON or wrong-shaped input fails with a diagnostic naming the file.
+#[test]
+fn coverage_aggregate_bad_input_fails() {
+    let tmp = tempfile::tempdir().expect("temp dir");
+    let bad = tmp.path().join("nope.json");
+    std::fs::write(&bad, "this is not json").expect("write");
+    let out = Command::new(rivet_bin())
+        .args(["coverage", "--aggregate", bad.to_str().unwrap()])
+        .output()
+        .expect("coverage --aggregate bad");
+    assert!(!out.status.success(), "must fail on bad input");
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(stderr.contains("nope.json"), "names the file. {stderr}");
+
+    let wrong = tmp.path().join("wrong.json");
+    std::fs::write(&wrong, r#"{"hello":"world"}"#).expect("write");
+    let out = Command::new(rivet_bin())
+        .args(["coverage", "--aggregate", wrong.to_str().unwrap()])
+        .output()
+        .expect("coverage --aggregate wrong");
+    assert!(!out.status.success(), "must fail on wrong shape");
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        stderr.contains("wrong.json") && stderr.contains("repos"),
+        "explains missing repos. {stderr}"
+    );
+}
+
 /// `rivet stats --format json` exposes diagnostic counts so consumers
 /// don't need a second `rivet validate --format json` call just to
 /// get the severity breakdown.