diff --git a/crates/socket-patch-cli/src/commands/remove.rs b/crates/socket-patch-cli/src/commands/remove.rs
index 3965a9ee2..f51222fa2 100644
--- a/crates/socket-patch-cli/src/commands/remove.rs
+++ b/crates/socket-patch-cli/src/commands/remove.rs
@@ -1295,9 +1295,15 @@ async fn revert_vendored_matches(
     let mut leg = RemoveVendorLeg::default();
     // Stops at the first hard failure: remove aborts there, leaving the
     // remaining matches (and the manifest) untouched.
-    let reverted = VendoredBackend::new(&args.common, None)
-        .revert(keys, state, opts, true)
-        .await;
+    let backend = VendoredBackend::new(&args.common, None);
+    let reverted = backend.revert(keys, state, opts, true).await;
+    for (key, w) in backend.go_undo_warnings(&reverted).await {
+        if loud {
+            eprintln!("Warning: {}", w.detail);
+        }
+        leg.skipped
+            .push(PatchEvent::new(PatchAction::Skipped, key).with_reason(w.code, w.detail));
+    }
     for RevertedEntry {
         key,
         warnings,
diff --git a/crates/socket-patch-cli/src/commands/rollback.rs b/crates/socket-patch-cli/src/commands/rollback.rs
index 6f98e9957..9a0c266d9 100644
--- a/crates/socket-patch-cli/src/commands/rollback.rs
+++ b/crates/socket-patch-cli/src/commands/rollback.rs
@@ -989,9 +989,14 @@ async fn run_vendored_leg(
         keep_artifact: preserve,
     };
     let loud = !common.json && !common.silent;
-    let reverted = VendoredBackend::new(common, None)
-        .revert(keys, state, opts, false)
-        .await;
+    let backend = VendoredBackend::new(common, None);
+    let reverted = backend.revert(keys, state, opts, false).await;
+    for (_, w) in backend.go_undo_warnings(&reverted).await {
+        if loud {
+            eprintln!("Warning: {}", w.detail);
+        }
+        out.warnings.push((w.code.to_string(), w.detail));
+    }
     for RevertedEntry {
         key,
         warnings,
diff --git a/crates/socket-patch-cli/src/commands/vendor.rs b/crates/socket-patch-cli/src/commands/vendor.rs
index bd85b661f..0bcb450f5 100644
--- a/crates/socket-patch-cli/src/commands/vendor.rs
+++ b/crates/socket-patch-cli/src/commands/vendor.rs
@@ -1241,7 +1241,8 @@ async fn run_check(args: &VendorArgs) -> i32 {
     } else {
         None
     };
-    let go_sync_issues = tokio::sync::OnceCell::new();
+    let go_sync_issues =
+        socket_patch_core::vendor::go_consumer_sync::audit(root, &HashMap::new()).await;
     for (key, entry) in entries {
         let record = entry.record.as_ref().or_else(|| manifest.patches.get(key));
         let mut failure = match record {
@@ -1280,11 +1281,6 @@ async fn run_check(args: &VendorArgs) -> i32 {
         if failure.is_none() && entry.ecosystem == "golang" {
             if let Some((module, _)) = socket_patch_core::utils::purl::parse_golang_purl(key) {
                 let issues: Vec<String> = go_sync_issues
-                    .get_or_init(|| async {
-                        socket_patch_core::vendor::go_consumer_sync::audit(root, &HashMap::new())
-                            .await
-                    })
-                    .await
                     .iter()
                     .filter(|issue| issue.module() == module)
                     .map(ToString::to_string)
@@ -1362,6 +1358,46 @@ async fn run_check(args: &VendorArgs) -> i32 {
                 .with_details(serde_json::json!({ "ecosystem": eco, "path": rel })),
         );
     }
+    // A committed vendor/modules.txt still recording a socket `replace`
+    // that go.mod no longer has breaks every default build until it is
+    // regenerated (#1777). The ledger entry it came from is gone after a
+    // revert, so the per-entry check above cannot see it.
+    let ledger_modules: HashSet<String> = state
+        .entries
+        .iter()
+        .filter(|(_, entry)| entry.ecosystem == "golang")
+        .filter_map(|(key, _)| socket_patch_core::utils::purl::parse_golang_purl(key))
+        .map(|(module, _)| module.into_owned())
+        .collect();
+    let stale_records: Vec<(String, String)> = go_sync_issues
+        .iter()
+        .filter(|issue| {
+            matches!(
+                issue,
+                socket_patch_core::vendor::go_consumer_sync::GoSyncIssue::VendorModulesTxt {
+                    stale: true,
+                    ..
+                }
+            ) && !ledger_modules.contains(issue.module())
+        })
+        .map(|issue| (issue.module().to_string(), issue.to_string()))
+        .collect();
+    for (module, detail) in stale_records {
+        if !args.common.json {
+            eprintln!(
+                "{}: {detail}",
+                socket_patch_core::vendor::go_consumer_sync::VENDOR_MODULES_TXT_CODE
+            );
+        }
+        env.record(
+            PatchEvent::artifact(PatchAction::Failed)
+                .with_error(
+                    socket_patch_core::vendor::go_consumer_sync::VENDOR_MODULES_TXT_CODE,
+                    detail,
+                )
+                .with_details(serde_json::json!({ "ecosystem": "golang", "module": module })),
+        );
+    }
     if args.common.json {
         println!("{}", env.to_pretty_json());
     }
@@ -4436,7 +4472,8 @@ async fn run_revert(args: &VendorArgs, env: &mut Envelope) -> i32 {
     // (rollback's vendored leg, both of remove's paths, the manifest
     // reconcile): dispatch → drift-keep → per-entry ledger save. Only the
     // event vocabulary and the human lines are this command's.
-    let reverted = VendoredBackend::new(common, None)
+    let backend = VendoredBackend::new(common, None);
+    let reverted = backend
         .revert(
             &recorded,
             &mut state,
@@ -4444,6 +4481,9 @@ async fn run_revert(args: &VendorArgs, env: &mut Envelope) -> i32 {
             false,
         )
         .await;
+    for (purl, w) in backend.go_undo_warnings(&reverted).await {
+        record_warning(env, &purl, &w, common);
+    }
     for RevertedEntry {
         key: purl,
         flavor,
diff --git a/crates/socket-patch-cli/src/commands/vendored_backend/mod.rs b/crates/socket-patch-cli/src/commands/vendored_backend/mod.rs
index 860e90809..a00d1e93c 100644
--- a/crates/socket-patch-cli/src/commands/vendored_backend/mod.rs
+++ b/crates/socket-patch-cli/src/commands/vendored_backend/mod.rs
@@ -199,6 +199,49 @@ impl<'a> VendoredBackend<'a> {
         }
         out
     }
+
+    /// Dropping a vendored Go `replace` leaves a committed
+    /// vendor/modules.txt still recording it, which breaks every default
+    /// build until `go mod vendor` runs (#343, #1777). The undo commands
+    /// call this after [`Self::revert`]: it audits the Go consumer state
+    /// once and returns the issues of the Go modules `reverted` unwired,
+    /// each keyed by the ledger key of its entry. Not part of `revert`
+    /// itself, whose forward-run caller (the manifest reconcile) audits
+    /// once at the end of the run instead.
+    pub(crate) async fn go_undo_warnings(
+        &self,
+        reverted: &[RevertedEntry],
+    ) -> Vec<(String, VendorWarning)> {
+        let unwired: Vec<(&str, String)> = reverted
+            .iter()
+            .filter(|e| {
+                matches!(
+                    e.step,
+                    VendorRevertStep::Reverted
+                        | VendorRevertStep::Preserved
+                        | VendorRevertStep::LedgerWriteFailed(_)
+                )
+            })
+            .filter_map(|e| {
+                socket_patch_core::utils::purl::parse_golang_purl(&e.key)
+                    .map(|(module, _)| (e.key.as_str(), module.into_owned()))
+            })
+            .collect();
+        if unwired.is_empty() {
+            return Vec::new();
+        }
+        socket_patch_core::vendor::go_consumer_sync::audit(&self.common.cwd, &HashMap::new())
+            .await
+            .into_iter()
+            .filter_map(|issue| {
+                let (key, _) = unwired.iter().find(|(_, m)| m == issue.module())?;
+                Some((
+                    key.to_string(),
+                    VendorWarning::new(issue.code(), issue.to_string()),
+                ))
+            })
+            .collect()
+    }
 }
 
 /// Name a patched `<name>-<version>.gem` left in Bundler's cache dir once
diff --git a/crates/socket-patch-cli/tests/e2e_vendor_golang_build.rs b/crates/socket-patch-cli/tests/e2e_vendor_golang_build.rs
index 852cfc0ef..bd772e205 100644
--- a/crates/socket-patch-cli/tests/e2e_vendor_golang_build.rs
+++ b/crates/socket-patch-cli/tests/e2e_vendor_golang_build.rs
@@ -1035,3 +1035,176 @@ fn go_vendor_reports_applied_event() {
 
     chmod_writable(tmp.path());
 }
+
+// ── #1777: vendored undo vs a committed vendor/ ───────────────────────
+
+/// #1777: in a project with a committed `vendor/`, a vendored patch plus
+/// the `go mod vendor` its forward warning asks for leaves
+/// `vendor/modules.txt` recording the socket `replace`. Every vendored undo
+/// (`vendor --revert`, `rollback`, `remove <purl>`) drops that `replace`, so
+/// the next default build fails with "inconsistent vendoring" until
+/// `go mod vendor` runs again. Each undo must name that regeneration (as
+/// agent-mode rollback does, #343), and `apply --check` must report the
+/// stale modules.txt as drift until it is run.
+fn vendored_undo_needs_go_mod_vendor(label: &str, undo: &[&str], workspace: bool) {
+    if !golang_e2e_matrix::toolchain_ready(&format!("e2e_vendor_golang_build({label})")) {
+        return;
+    }
+    let tmp = tempfile::tempdir().unwrap();
+    let (consumer, modcache, proxy) = stage(tmp.path());
+    let mc = modcache.to_str().unwrap();
+    let mut goenv = go_env(mc, &proxy);
+    // In a workspace, `go work vendor` owns vendor/modules.txt.
+    let regen_cmd: &[&str] = if workspace {
+        std::fs::write(consumer.join("go.work"), "go 1.21\n\nuse .\n").unwrap();
+        &["work", "vendor"]
+    } else {
+        &["mod", "vendor"]
+    };
+    if workspace {
+        // `-mod=mod` is not allowed in workspace mode.
+        goenv.retain(|(k, _)| *k != "GOFLAGS");
+        goenv.push(("GOFLAGS", ""));
+    }
+    let vendored = go(&consumer, regen_cmd, &goenv);
+    assert!(
+        vendored.status.success(),
+        "{regen_cmd:?}: {}",
+        String::from_utf8_lossy(&vendored.stderr)
+    );
+    // Default flags from here on: go builds from vendor/.
+    goenv.retain(|(k, _)| *k != "GOFLAGS");
+    goenv.push(("GOFLAGS", ""));
+
+    write_patch(&consumer);
+    let (code, stdout, stderr) = run_socket(
+        &consumer,
+        &[
+            "vendor",
+            "--json",
+            "--offline",
+            "--cwd",
+            consumer.to_str().unwrap(),
+        ],
+        &modcache,
+    );
+    assert_eq!(
+        code, 0,
+        "vendor failed.\nstdout:\n{stdout}\nstderr:\n{stderr}"
+    );
+    let revendor = go(&consumer, regen_cmd, &goenv);
+    assert!(
+        revendor.status.success(),
+        "{}",
+        String::from_utf8_lossy(&revendor.stderr)
+    );
+    let run = go(&consumer, &["run", "."], &goenv);
+    assert!(
+        String::from_utf8_lossy(&run.stdout).contains("OUT: PATCHED"),
+        "{}{}",
+        String::from_utf8_lossy(&run.stdout),
+        String::from_utf8_lossy(&run.stderr)
+    );
+
+    {
+        let cs = consumer.to_str().unwrap();
+        let mut args = undo.to_vec();
+        args.extend(["--cwd", cs]);
+        let (code, so, se) = run_socket(&consumer, &args, &modcache);
+        assert_eq!(code, 0, "[{label}] failed.\n{so}\n{se}");
+        let renv = parse_json_envelope(&so);
+        assert!(
+            !std::fs::read_to_string(consumer.join("go.mod"))
+                .unwrap()
+                .contains("replace "),
+            "[{label}] the socket replace is gone"
+        );
+        let broken = go(&consumer, &["build", "./..."], &goenv);
+        assert!(
+            !broken.status.success()
+                && String::from_utf8_lossy(&broken.stderr).contains("inconsistent vendoring"),
+            "[{label}] the fixture reproduces the broken build: {}",
+            String::from_utf8_lossy(&broken.stderr)
+        );
+        assert!(
+            common::envelope::all_codes(&renv)
+                .contains(&"go_vendor_modules_txt_out_of_sync".to_string()),
+            "[{label}] must name the vendor/modules.txt regeneration:\n{renv}"
+        );
+        let (code, so, se) = run_socket(
+            &consumer,
+            &["apply", "--check", "--ecosystems", "golang", "--cwd", cs],
+            &modcache,
+        );
+        assert_eq!(
+            code, 1,
+            "[{label}] stale vendor/modules.txt is drift:\n{so}\n{se}"
+        );
+        let named = format!("go {}", regen_cmd.join(" "));
+        assert!(se.contains(&named), "[{label}] must name `{named}`: {se}");
+        let (code, so, se) = run_socket(&consumer, &["vendor", "--check", "--cwd", cs], &modcache);
+        assert_eq!(
+            code, 1,
+            "[{label}] vendor --check: stale vendor/modules.txt is drift:\n{so}\n{se}"
+        );
+        assert!(se.contains(&named), "[{label}] must name `{named}`: {se}");
+
+        // The step the warning names fixes the build.
+        let regen = go(&consumer, regen_cmd, &goenv);
+        assert!(
+            regen.status.success(),
+            "[{label}] {}",
+            String::from_utf8_lossy(&regen.stderr)
+        );
+        let run = go(&consumer, &["run", "."], &goenv);
+        assert!(
+            String::from_utf8_lossy(&run.stdout).contains("OUT: PRISTINE"),
+            "[{label}] {}{}",
+            String::from_utf8_lossy(&run.stdout),
+            String::from_utf8_lossy(&run.stderr)
+        );
+        // (`vendor --revert` keeps the manifest record, which `--check`
+        // still reports as unledgered; only the modules.txt drift is gone.)
+        let (_code, so, se) = run_socket(&consumer, &["vendor", "--check", "--cwd", cs], &modcache);
+        assert!(
+            !se.contains("go_vendor_modules_txt_out_of_sync") && !so.contains("modules.txt"),
+            "[{label}] in sync after {named}:\n{so}\n{se}"
+        );
+    }
+
+    chmod_writable(tmp.path());
+}
+
+const REVERT: &[&str] = &["vendor", "--revert", "--json"];
+const ROLLBACK: &[&str] = &["rollback", "--offline", "--json"];
+const REMOVE: &[&str] = &["remove", UPURL, "--offline", "--json"];
+
+#[test]
+fn vendor_revert_needs_go_mod_vendor_with_committed_vendor_dir() {
+    vendored_undo_needs_go_mod_vendor("vendor --revert", REVERT, false);
+}
+
+#[test]
+fn vendored_rollback_needs_go_mod_vendor_with_committed_vendor_dir() {
+    vendored_undo_needs_go_mod_vendor("rollback", ROLLBACK, false);
+}
+
+#[test]
+fn vendored_remove_needs_go_mod_vendor_with_committed_vendor_dir() {
+    vendored_undo_needs_go_mod_vendor("remove", REMOVE, false);
+}
+
+#[test]
+fn vendor_revert_needs_go_work_vendor_in_workspace() {
+    vendored_undo_needs_go_mod_vendor("go.work vendor --revert", REVERT, true);
+}
+
+#[test]
+fn vendored_rollback_needs_go_work_vendor_in_workspace() {
+    vendored_undo_needs_go_mod_vendor("go.work rollback", ROLLBACK, true);
+}
+
+#[test]
+fn vendored_remove_needs_go_work_vendor_in_workspace() {
+    vendored_undo_needs_go_mod_vendor("go.work remove", REMOVE, true);
+}
diff --git a/docs/ecosystems.md b/docs/ecosystems.md
index 62b0b8556..18063b65b 100644
--- a/docs/ecosystems.md
+++ b/docs/ecosystems.md
@@ -1475,8 +1475,9 @@ for it until `rollback -g`.
 
 Two files the go command derives from `go.mod` can disagree with a new or removed
 socket `replace`, and go then refuses to build. socket-patch does not regenerate
-them (that needs the go toolchain and the whole module graph); `apply`, `vendor`,
-`rollback` and `scan --mode hosted` warn with the command that does, and
+them (that needs the go toolchain and the whole module graph); `apply`, `vendor`
+(including `vendor --revert`), `rollback`, `remove` and `scan --mode hosted` warn
+with the command that does, and
 `apply --check` / `vendor --check` report the disagreement as drift until it is run:
 
 | Warning code | When | Fix |