diff --git a/README.md b/README.md
index b7bf170..9f3a12e 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,20 @@
 2. be gentle, preferably don't run mass rebuilds / massive builds like
    chromium on it
 
+## Automatic Building
+
+Users who are _trusted_ (see: ./config.public.json) will have their
+PRs automatically trigger builds if their commits follow the
+well-defined format of Nixpkgs. Example messages and the builds:
+
+|Message|Automatic Build|
+|-|-|
+|`vim: 1.0.0 -> 2.0.0`|`vim`|
+|`python3Packages.requests,python2Packages.requests: 1.0.0 -> 2.0.0`|`python3Packages.requests`, `python2Packages.requests`|
+|`python{2,3}Packages.requests: 1.0.0 -> 2.0.0`|_nothing_|
+
+
+
 ## Commands
 
 The comment parser is line-based, so comments can be interleaved with
diff --git a/ofborg/src/acl.rs b/ofborg/src/acl.rs
index f1a9d3e..66e8d83 100644
--- a/ofborg/src/acl.rs
+++ b/ofborg/src/acl.rs
@@ -12,6 +12,20 @@ impl ACL {
         };
     }
 
+    pub fn build_job_destinations_for_user_repo(&self, user: &str, repo: &str)
+                                            -> Vec<(Option<String>, Option<String>)> {
+        if self.can_build_unrestricted(user, repo) {
+            vec![(Some("build-jobs".to_owned()), None)]
+        } else if self.can_build_restricted(user, repo) {
+            vec![
+                (None, Some("build-inputs-x86_64-linux".to_owned())),
+                (None, Some("build-inputs-aarch64-linux".to_owned())),
+            ]
+        } else {
+            vec![]
+        }
+    }
+
     pub fn can_build_restricted(&self, user: &str, repo: &str) -> bool {
         if repo.to_lowercase() != "nixos/nixpkgs" {
             return false;
diff --git a/ofborg/src/bin/mass-rebuilder.rs b/ofborg/src/bin/mass-rebuilder.rs
index 0810fad..79732d2 100644
--- a/ofborg/src/bin/mass-rebuilder.rs
+++ b/ofborg/src/bin/mass-rebuilder.rs
@@ -36,6 +36,7 @@ fn main() {
         cloner,
         nix,
         cfg.github(),
+        cfg.acl(),
         cfg.runner.identity.clone(),
         events,
     );
diff --git a/ofborg/src/checkout.rs b/ofborg/src/checkout.rs
index 98aaedf..040a01d 100644
--- a/ofborg/src/checkout.rs
+++ b/ofborg/src/checkout.rs
@@ -147,6 +147,32 @@ impl CachedProjectCo {
             return Err(Error::new(ErrorKind::Other, "Failed to merge"));
         }
     }
+
+
+    pub fn commit_messages_from_head(&self, commit: &str) -> Result<Vec<String>, Error> {
+        let mut lock = self.lock()?;
+
+        let result = Command::new("git")
+            .arg("log")
+            .arg("--format=format:%s")
+            .arg(format!("HEAD..{}", commit))
+            .current_dir(self.clone_to())
+            .output()?;
+
+        lock.unlock();
+
+        if result.status.success() {
+            return Ok(
+                String::from_utf8_lossy(&result.stdout)
+                    .lines()
+                    .map(|l| l.to_owned())
+                    .collect()
+            );
+        } else {
+            return Err(Error::new(ErrorKind::Other,
+                                  String::from_utf8_lossy(&result.stderr).to_lowercase()));
+        }
+    }
 }
 
 impl clone::GitClonable for CachedProjectCo {
@@ -197,3 +223,52 @@ impl clone::GitClonable for CachedProject {
         return vec![OsStr::new("--bare")];
     }
 }
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::path::{Path, PathBuf};
+    use std::process::{Command, Stdio};
+    use ofborg::test_scratch::TestScratch;
+
+    fn tpath(component: &str) -> PathBuf {
+        return Path::new(env!("CARGO_MANIFEST_DIR")).join(component);
+    }
+
+    fn make_pr_repo(bare: &Path, co: &Path) -> String {
+        let output = Command::new("./make-pr.sh")
+            .current_dir(tpath("./test-srcs"))
+            .arg(bare)
+            .arg(co)
+            .stderr(Stdio::null())
+            .stdout(Stdio::piped())
+            .output()
+            .expect("building the test PR failed");
+        let hash = String::from_utf8(output.stdout).expect("Should just be a hash");
+        return hash.trim().to_owned();
+    }
+
+    #[test]
+    pub fn test_commit_msg_list() {
+        let workingdir = TestScratch::new_dir("test-test-commit-msg-list");
+
+        let bare = TestScratch::new_dir("bare-commit-messages");
+        let mk_co = TestScratch::new_dir("mk-commit-messages");
+        let hash = make_pr_repo(&bare.path(), &mk_co.path());
+
+        let cloner = cached_cloner(&workingdir.path());
+        let project = cloner.project("commit-msg-list".to_owned(), bare.string());
+        let working_co = project.clone_for("testing-commit-msgs".to_owned(), "123".to_owned()).expect("clone should work");
+        working_co.checkout_origin_ref(OsStr::new("master"));
+
+        let expect: Vec<String> = vec![
+            "check out this cool PR".to_owned()
+        ];
+
+        assert_eq!(
+            working_co.commit_messages_from_head(&hash).expect("fetching messages should work"),
+            expect
+        );
+    }
+}
diff --git a/ofborg/src/message/buildjob.rs b/ofborg/src/message/buildjob.rs
index 81599fe..9930af8 100644
--- a/ofborg/src/message/buildjob.rs
+++ b/ofborg/src/message/buildjob.rs
@@ -8,8 +8,37 @@ pub struct BuildJob {
     pub pr: Pr,
     pub subset: Option<Subset>,
     pub attrs: Vec<String>,
-    pub logs: Option<(Option<String>, Option<String>)>, // (Exchange, Routing Key)
-    pub statusreport: Option<(Option<String>, Option<String>)>, // (Exchange, Routing Key)
+    pub logs: Option<ExchangeQueue>, // (Exchange, Routing Key)
+    pub statusreport: Option<ExchangeQueue>, // (Exchange, Routing Key)
+}
+
+pub type ExchangeQueue = (Option<Exchange>, Option<RoutingKey>);
+type Exchange = String;
+type RoutingKey = String;
+
+impl BuildJob {
+    pub fn new(repo: Repo,
+               pr: Pr,
+               subset: Subset,
+               attrs: Vec<String>,
+               logs: Option<ExchangeQueue>,
+               statusreport: Option<ExchangeQueue>) -> BuildJob {
+
+        let logbackrk = format!(
+            "{}.{}",
+            repo.full_name.clone(),
+            pr.number,
+        ).to_lowercase();
+
+        BuildJob {
+            repo: repo,
+            pr: pr,
+            subset: Some(subset),
+            attrs: attrs,
+            logs: Some(logs.unwrap_or((Some("logs".to_owned()), Some(logbackrk)))),
+            statusreport: Some(statusreport.unwrap_or((Some("build-results".to_owned()), None))),
+        }
+    }
 }
 
 pub fn from(data: &Vec<u8>) -> Result<BuildJob, serde_json::error::Error> {
diff --git a/ofborg/src/message/massrebuildjob.rs b/ofborg/src/message/massrebuildjob.rs
index a486bca..ae109a1 100644
--- a/ofborg/src/message/massrebuildjob.rs
+++ b/ofborg/src/message/massrebuildjob.rs
@@ -20,7 +20,8 @@ impl Actions {
         return vec![worker::Action::Ack];
     }
 
-    pub fn done(&mut self, _job: &MassRebuildJob) -> worker::Actions {
-        return vec![worker::Action::Ack];
+    pub fn done(&mut self, _job: &MassRebuildJob, mut response: worker::Actions) -> worker::Actions {
+        response.push(worker::Action::Ack);
+        return response;
     }
 }
diff --git a/ofborg/src/tasks/githubcommentfilter.rs b/ofborg/src/tasks/githubcommentfilter.rs
index 55c54c8..5e3fa31 100644
--- a/ofborg/src/tasks/githubcommentfilter.rs
+++ b/ofborg/src/tasks/githubcommentfilter.rs
@@ -53,30 +53,13 @@ impl worker::SimpleWorker for GitHubCommentWorker {
             return vec![worker::Action::Ack];
         }
 
-        let build_destinations: Vec<(Option<String>, Option<String>)>;
+        let build_destinations = self.acl.build_job_destinations_for_user_repo(
+            &job.comment.user.login,
+            &job.repository.full_name,
+        );
 
-        if self.acl.can_build_unrestricted(
-            &job.comment.user.login,
-            &job.repository.full_name,
-        )
-        {
-            build_destinations = vec![(Some("build-jobs".to_owned()), None)];
-        } else if self.acl.can_build_restricted(
-            &job.comment.user.login,
-            &job.repository.full_name,
-        )
-        {
-            build_destinations = vec![
-                (None, Some("build-inputs-x86_64-linux".to_owned())),
-                (None, Some("build-inputs-aarch64-linux".to_owned())),
-            ];
-        } else {
-            println!(
-                "ACL prohibits {} from building {:?} for {}",
-                job.comment.user.login,
-                instructions,
-                job.repository.full_name
-            );
+        if build_destinations.len() == 0 {
+            // Don't process comments if they can't build anything
             return vec![worker::Action::Ack];
         }
 
@@ -124,19 +107,14 @@ impl worker::SimpleWorker for GitHubCommentWorker {
             for instruction in instructions {
                 match instruction {
                     commentparser::Instruction::Build(subset, attrs) => {
-                        let logbackrk = format!(
-                            "{}.{}",
-                            job.repository.full_name.clone(),
-                            job.issue.number.clone()
+                        let msg = buildjob::BuildJob::new(
+                            repo_msg.clone(),
+                            pr_msg.clone(),
+                            subset,
+                            attrs,
+                            None,
+                            None
                         );
-                        let msg = buildjob::BuildJob {
-                            repo: repo_msg.clone(),
-                            pr: pr_msg.clone(),
-                            subset: Some(subset),
-                            attrs: attrs,
-                            logs: Some((Some("logs".to_owned()), Some(logbackrk.to_lowercase()))),
-                            statusreport: Some((Some("build-results".to_owned()), None)),
-                        };
 
                         for (exch, rk) in build_destinations.clone() {
                             response.push(worker::publish_serde_action(exch, rk, &msg));
diff --git a/ofborg/src/tasks/massrebuilder.rs b/ofborg/src/tasks/massrebuilder.rs
index 9411742..843af5c 100644
--- a/ofborg/src/tasks/massrebuilder.rs
+++ b/ofborg/src/tasks/massrebuilder.rs
@@ -9,15 +9,17 @@ use std::io::BufReader;
 use std::path::Path;
 use std::path::PathBuf;
 use ofborg::checkout;
-use ofborg::message::massrebuildjob;
+use ofborg::message::{massrebuildjob, buildjob};
 use ofborg::nix::Nix;
 
+use ofborg::acl::ACL;
 use ofborg::stats;
 use ofborg::worker;
 use ofborg::tagger::{StdenvTagger, RebuildTagger};
 use ofborg::outpathdiff::{OutPaths, OutPathDiff};
 use ofborg::evalchecker::EvalChecker;
 use ofborg::commitstatus::CommitStatus;
+use ofborg::commentparser::Subset;
 use amqp::protocol::basic::{Deliver, BasicProperties};
 use hubcaps;
 
@@ -25,6 +27,7 @@ pub struct MassRebuildWorker<E> {
     cloner: checkout::CachedCloner,
     nix: Nix,
     github: hubcaps::Github,
+    acl: ACL,
     identity: String,
     events: E,
 }
@@ -34,6 +37,7 @@ impl<E: stats::SysEvents> MassRebuildWorker<E> {
         cloner: checkout::CachedCloner,
         nix: Nix,
         github: hubcaps::Github,
+        acl: ACL,
         identity: String,
         events: E,
     ) -> MassRebuildWorker<E> {
@@ -41,6 +45,7 @@ impl<E: stats::SysEvents> MassRebuildWorker<E> {
             cloner: cloner,
             nix: nix.without_limited_supported_systems(),
             github: github,
+            acl: acl,
             identity: identity,
             events: events,
         };
@@ -86,6 +91,8 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
         let gists = self.github.gists();
         let issue = repo.issue(job.pr.number);
 
+        let auto_schedule_build_archs: Vec<buildjob::ExchangeQueue>;
+
         match issue.get() {
             Ok(iss) => {
                 if iss.state == "closed" {
@@ -93,6 +100,11 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
                     info!("Skipping {} because it is closed", job.pr.number);
                     return self.actions().skip(&job);
                 }
+
+                auto_schedule_build_archs = self.acl.build_job_destinations_for_user_repo(
+                    &iss.user.login,
+                    &job.repo.full_name,
+                );
             }
             Err(e) => {
                 self.events.tick("issue-fetch-failed");
@@ -182,6 +194,10 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
             return self.actions().skip(&job);
         }
 
+        let possibly_touched_packages = parse_commit_messages(
+            co.commit_messages_from_head(&job.pr.head_sha).unwrap_or(vec!["".to_owned()])
+        );
+
         overall_status.set_with_description("Merging PR", hubcaps::statuses::State::Pending);
 
         if let Err(_) = co.merge_commit(job.pr.head_sha.as_ref()) {
@@ -341,6 +357,9 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
             })
             .all(|status| status == Ok(()));
 
+
+        let mut response: worker::Actions = vec![];
+
         if eval_results {
             let mut status = CommitStatus::new(
                 repo.statuses(),
@@ -357,9 +376,37 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
 
             let checker = OutPaths::new(self.nix.clone(), PathBuf::from(&refpath), true);
             match checker.find() {
-                Ok(_) => {
+                Ok(pkgs) => {
                     state = hubcaps::statuses::State::Success;
                     gist_url = None;
+
+                    let mut try_build: Vec<String> = pkgs
+                        .keys()
+                        .map(|pkgarch| pkgarch.package.clone())
+                        .filter(|pkg| {
+                            possibly_touched_packages.contains(&pkg)
+                        })
+                        .collect();
+                    try_build.sort();
+                    try_build.dedup();
+
+                    if try_build.len() <= 10 {
+                        // In the case of trying to merge master in to
+                        // a stable branch, we don't want to do this.
+                        // Therefore, only schedule builds if there
+                        // less than or exactly 10
+                        let msg = buildjob::BuildJob::new(
+                            job.repo.clone(),
+                            job.pr.clone(),
+                            Subset::Nixpkgs,
+                            try_build,
+                            None,
+                            None,
+                        );
+                        for (dest, rk) in auto_schedule_build_archs {
+                            response.push(worker::publish_serde_action(dest, rk, &msg));
+                        }
+                    }
                 }
                 Err(mut out) => {
                     eval_results = false;
@@ -412,7 +459,7 @@ impl<E: stats::SysEvents> worker::SimpleWorker for MassRebuildWorker<E> {
             );
         }
 
-        return self.actions().done(&job);
+        return self.actions().done(&job, response);
     }
 }
 
@@ -619,6 +666,24 @@ fn file_to_str(f: &mut File) -> String {
     return String::from(String::from_utf8_lossy(&buffer));
 }
 
+fn parse_commit_messages(messages: Vec<String>) -> Vec<String> {
+    messages
+        .iter()
+        .filter_map(|line| {
+            // Convert "foo: some notes" in to "foo"
+            let parts: Vec<&str> = line.splitn(2, ":").collect();
+            if parts.len() == 2 {
+                Some(parts[0])
+            } else {
+                None
+            }
+        })
+        .flat_map(|line| { let pkgs: Vec<&str> = line.split(",").collect(); pkgs })
+        .map(|line| line.trim().to_owned())
+
+        .collect()
+}
+
 #[cfg(test)]
 mod tests {
 
@@ -640,4 +705,36 @@ mod tests {
 
         assert!(stdenv.are_same());
     }
+
+    #[test]
+    fn test_parse_commit_messages() {
+        let expect: Vec<&str> = vec![
+            "firefox{-esr", // don't support such fancy syntax
+            "}", // Don't support such fancy syntax
+            "firefox",
+            "buildkite-agent",
+            "python.pkgs.ptyprocess",
+            "python.pkgs.ptyprocess",
+            "android-studio-preview",
+            "foo",
+            "bar",
+        ];
+        assert_eq!(
+            parse_commit_messages("
+              firefox{-esr,}: fix failing build due to the google-api-key
+              Merge pull request #34483 from andir/dovecot-cve-2017-15132
+              firefox: enable official branding
+              Merge pull request #34442 from rnhmjoj/virtual
+              buildkite-agent: enable building on darwin
+              python.pkgs.ptyprocess: 0.5 -> 0.5.2
+              python.pkgs.ptyprocess: move expression
+              Merge pull request #34465 from steveeJ/steveej-attempt-qtile-bump-0.10.7
+              android-studio-preview: 3.1.0.8 -> 3.1.0.9
+              Merge pull request #34188 from dotlambda/home-assistant
+              Merge pull request #34414 from dotlambda/postfix
+              foo,bar: something here: yeah
+            ".lines().map(|l| l.to_owned()).collect()),
+            expect
+        );
+    }
 }
diff --git a/ofborg/src/test_scratch.rs b/ofborg/src/test_scratch.rs
index 4eb7418..cec800b 100644
--- a/ofborg/src/test_scratch.rs
+++ b/ofborg/src/test_scratch.rs
@@ -34,6 +34,10 @@ impl TestScratch {
     pub fn path(&self) -> PathBuf {
         self.root.clone()
     }
+
+    pub fn string(&self) -> String {
+        self.path().to_str().unwrap().to_owned()
+    }
 }
 
 impl Drop for TestScratch {