diff --git a/FU.pm b/FU.pm
index 25db727..d5ce879 100644
--- a/FU.pm
+++ b/FU.pm
@@ -130,21 +130,27 @@ sub init_db($info) {
 }
 
 
-my @before_request;
-my @after_request;
-sub before_request :prototype(&) ($f) { push @before_request, $f }
-sub after_request :prototype(&) ($f) { unshift @after_request, $f }
+sub _caller_info {
+    my($i, @c, @x) = (1);
+    $x[0] !~ /^FU(?:$|::)/ && push @c, [ @x[0..3] ] while (@x = caller $i++);
+    \@c
+}
+
+our @before_request;
+our @after_request;
+sub before_request :prototype(&) ($f) { push @before_request, [ $f, _caller_info ] }
+sub after_request :prototype(&) ($f) { unshift @after_request, [ $f, _caller_info ] }
 
 
-my %path_routes;
-my %re_routes;
+our %path_routes;
+our %re_routes;
 
 sub _add_route($path, $sub, $method) {
     if (ref $path eq 'REGEXP' || ref $path eq 'Regexp') {
-        push $re_routes{$method}->@*, [ qr/^$path$/, $sub ];
+        push $re_routes{$method}->@*, [ qr/^$path$/, $sub, _caller_info ];
     } elsif (!ref $path) {
         confess("A route has already been registered for $method $path") if $path_routes{$method}{$path};
-        $path_routes{$method}{$path} = $sub;
+        $path_routes{$method}{$path} = [ $sub, _caller_info ];
     } else {
         confess('Path argument in route registration must be a string or regex');
     }
@@ -207,6 +213,12 @@ sub _monitor {
 }
 
 
+our $debug_info = [];
+sub debug_info($path, $storage=undef, $history=100) {
+    $debug_info = { path => $path, storage => $storage, history => $history }
+}
+
+
 our $hdrname_re = qr/[!#\$\%&'\*\+-\.^_`\|~0-9a-zA-Z]{1,127}/;
 our $method_re = qr/(?:HEAD|GET|POST|DELETE|OPTIONS|PUT|PATCH|QUERY)/;
 
@@ -305,7 +317,7 @@ sub _log_err($e) {
 }
 
 sub _do_req($c) {
-    local $REQ = { hdr => {}, trace_start => time };
+    local $REQ = { hdr => {}, trace_start => time, trace_id => sprintf('%010x%08x%04x', int time, $$, int rand 1<<16) };
     local $fu = bless {}, 'FU::obj';
 
     $REQ->{ip} = $c->{client_sock} isa 'IO::Socket::INET' ? $c->{client_sock}->peerhost : '127.0.0.1';
@@ -315,15 +327,27 @@ sub _do_req($c) {
         _read_req $c;
         $REQ->{trace_start} = time;
 
-        for my $h (@before_request) { $h->() }
-
         my $path = fu->path;
         my $method = fu->method eq 'HEAD' ? 'GET' : fu->method;
+
+        # Intercept requests for debug_info, ensuring no website hooks get called.
+        if (debug && $method eq 'GET' && $debug_info->{path} && $path eq $debug_info->{path}) {
+            require FU::DebugImpl;
+            FU::DebugImpl::render();
+            fu->_flush($c->{fcgi_obj} || $c->{client_sock});
+            fu->error(-1);
+        }
+
+        for my $h (@before_request) { $h->[0]->() }
+
         my $r = $path_routes{$method}{$path};
-        if ($r) { $r->() }
-        else {
+        if ($r) {
+            $REQ->{trace_han} = [ $path, $r->[1] ];
+            $r->[0]->();
+        } else {
             for $r ($re_routes{ fu->method }->@*) {
                 if($path =~ $r->[0]) {
+                    $REQ->{trace_han} = [ $r->[0], $r->[2] ];
                     $r->[1]->(@{^CAPTURE});
                     fu->done;
                 }
@@ -333,11 +357,12 @@ sub _do_req($c) {
         1;
     };
     return if !$ok && ref $@ eq 'FU::err' && $@->[0] == -1;
+    $REQ->{trace_exn} = $ok ? undef : $@;
     my $err = $ok || _is_done($@) ? undef : $@;
     _log_err $err;
 
     for my $h (@after_request) {
-        $ok = eval { $h->(); 1 };
+        $ok = eval { $h->[0]->(); 1 };
         _log_err $@ if !$ok;
         $err = $@ if !$err && !$ok && !_is_done($@);
     }
@@ -361,7 +386,12 @@ sub _do_req($c) {
     $REQ->{trace_end} = time;
     fu->_flush($c->{fcgi_obj} || $c->{client_sock});
 
-    my $proc_ms = (time - $REQ->{trace_start}) * 1000;
+    if (debug && $REQ->{trace_id} && $debug_info->{history} && $debug_info->{storage}) {
+        require FU::DebugImpl;
+        FU::DebugImpl::save();
+    }
+
+    my $proc_ms = ($REQ->{trace_end} - $REQ->{trace_start}) * 1000;
     log_write(sprintf "%.0fms%s %s-%s %s-%d\n", $proc_ms,
         $REQ->{trace_nsql} ?
             sprintf ' (sql %.0f+%.0fms, %d/%d/%d)',
@@ -639,11 +669,15 @@ sub formdata {
 
 # Response generation methods
 
-sub done { die bless [200,'Done'], 'FU::err' }
-sub error($,$code,$msg=$code) { die bless [$code,$msg], 'FU::err' }
+sub done { die bless [200,'Done',FU::_caller_info], 'FU::err' }
+sub error($,$code,$msg=$code) { die bless [$code,$msg,FU::_caller_info], 'FU::err' }
 
 sub status($, $code) { $FU::REQ->{status} = $code }
-sub set_body($, $data) { $FU::REQ->{resbody} = $data }
+sub set_body($, $data) {
+    confess "Invalid undef body" if !defined $data;
+    confess "Invalid attempt to set body to $data" if ref $data;
+    $FU::REQ->{resbody} = $data;
+}
 
 sub reset {
     fu->status(200);
@@ -950,9 +984,29 @@ handling and performance tracing.
 Enable or disable debug mode. Returns the current mode when no argument is
 given.
 
-Debug mode currently only enables more verbose logging, but it may influence
-other features in the future as well. You're of course free to use the debug
-setting to enable or disable debugging features in your own code.
+Debug mode currently enables more verbose logging and the C<debug_info>
+interface below. It may influence other features in the future as well. You're
+of course free to use the debug setting to enable or disable debugging features
+in your own code.
+
+=item FU::debug_info($path, $storage, $history)
+
+Enable the built-in web interface for inspecting debug info. The interface is
+accessible from your browser at the given C<$path>, which is matched against
+C<< fu->path >>.
+
+When the optional C<$storage> argument is given and set to an existing
+directory, detailed request data is logged and stored in that directory, which
+is then made available through the web interface. The C<$history> argument sets
+the number of requests to keep, which defaults to 100.
+
+Request logging and the web interface are only available when C<FU::debug> mode
+is enabled.
+
+B<WARNING:> This interface exposes internal and potentially sensitive
+information. When this option is configured, make sure to B<ABSOLUTELY NEVER>
+enable debug mode in production! Or at least set an absolutely impossible to
+guess C<$path>.
 
 =item FU::log_slow_reqs($ms)
 
@@ -1271,7 +1325,7 @@ though.
 
 This method loads the entire file contents in memory and does not support range
 requests, so DO NOT use it to send large files. Actual web servers are much
-more efficient at sending static files.
+more efficient at serving static files.
 
 The content-type header is determined from the file extension in C<$path>,
 using the configured C<FU::mime_types>. As fallback, files that look like they
@@ -1424,7 +1478,7 @@ external process manager.
 
 =back
 
-When C<--monitor> or C<--max-reqs> are set or C<<--proc>> is larger than 1, FU
+When C<--monitor> or C<--max-reqs> are set or C<--proc> is larger than 1, FU
 starts a supervisor process to ensure the requested number of worker processes
 are running and that they are restarted when necessary. When FU has been loaded
 with the C<-spawn> flag, this supervisor process runs directly from the context
diff --git a/FU/DebugImpl.pm b/FU/DebugImpl.pm
new file mode 100644
index 0000000..1c1875e
--- /dev/null
+++ b/FU/DebugImpl.pm
@@ -0,0 +1,353 @@
+# Internal module used by FU.pm
+package FU::DebugImpl 0.1;
+use v5.36;
+use experimental 'for_list';
+use FU;
+use FU::XMLWriter ':html5_', 'fragment', 'xml_escape';
+use Time::HiRes 'time';
+use POSIX 'strftime';
+
+sub fmtts { strftime '%Y-%m-%d %H:%M:%S UTC', gmtime shift }
+
+sub loc_($loc) {
+    txt_ '[internal]' if !@$loc;
+    for (0..$#$loc) {
+        br_ if $_;
+        my $l = $loc->[$_];
+        my $f = $_ == $#$loc ? '(main)' : $loc->[$_+1][3];
+        txt_ "$l->[1]:$l->[2] $f";
+    }
+}
+
+sub fmtpre_($code) {
+    lit_ xml_escape($code) =~ s/^\s+//r =~ s/\s+$//r =~ s/\n/<br>/rg;
+}
+
+sub clean_re($str) {
+    # Regex formatting isn't stable, but this cleans up the crap I'm seeing a little bit.
+    "$str" =~ s/^\(\?\^u:\^\(\?\^u://r =~ s/\)\$\)$//r;
+}
+
+my @tabs = (
+    req => sub {
+        table_ sub {
+            tr_ sub { td_ 'Method'; td_ fu->method };
+            tr_ sub { td_ 'Path'; td_ fu->path };
+            tr_ sub { td_ 'Query'; td_ fu->query };
+            tr_ sub { td_ 'Client IP'; td_ fu->ip };
+            tr_ sub { td_ 'Received'; td_ fmtts $FU::REQ->{trace_start} };
+        };
+        h2_ 'Headers';
+        table_ sub {
+            tr_ sub {
+                td_ $_;
+                td_ fu->headers->{$_};
+            } for sort keys fu->headers->%*;
+        };
+        h2_ 'Body';
+        p_ 'TODO';
+        ('Request')
+    },
+
+    res => sub {
+        my $r = $FU::REQ;
+        return () if !exists $r->{trace_end};
+        table_ sub {
+            tr_ sub { td_ 'Status'; td_ $r->{status} };
+            tr_ sub {
+                td_ 'Handler';
+                td_ $r->{trace_han} ? sub {
+                    txt_ clean_re $r->{trace_han}[0];
+                    br_;
+                    loc_ $r->{trace_han}[1];
+                } : 'N/A';
+            };
+            my $exn = $r->{trace_exn};
+            tr_ sub {
+                td_ 'Exception';
+                td_ !defined $exn ? 'N/A' : ref $exn eq 'FU::err' ? sub {
+                    txt_ $exn->[0];
+                    txt_ " $exn->[1]" if $exn->[1] ne $exn->[0];
+                    br_;
+                    loc_ $exn->[2];
+                } : $exn;
+            };
+            tr_ sub { td_ 'Timing'; td_ sprintf '%.1f ms', ($r->{trace_end}-$r->{trace_start})*1000 };
+        };
+        h2_ 'Headers';
+        table_ sub {
+            tr_ sub {
+                td_ $_;
+                td_ $r->{reshdr}{$_};
+            } for keys $r->{reshdr}->%*;
+        };
+        ('Response')
+    },
+
+    sql => sub {
+        return () if !$FU::REQ->{trace_sql};
+        table_ sub {
+            thead_ sub { tr_ sub {
+                td_ class => 'num', 'Exec';
+                td_ class => 'num', 'Prep';
+                td_ class => 'num', 'Rows';
+                td_ 'Query';
+            } };
+            tr_ sub {
+                td_ class => 'num', sprintf '%.1f ms', $_->{exec_time}*1000;
+                td_ class => 'num', !defined $_->{prepare_time} ? '-' : $_->{prepare_time} ? sprintf '%.1f ms', $_->{prepare_time}*1000 : 'cache';
+                td_ class => 'num', $_->{nrows};
+                td_ class => 'code', sub { fmtpre_ $_->{query} };
+                # TODO: Params, both separate and interpolated
+            } for $FU::REQ->{trace_sql}->@*;
+        };
+        ('Queries', scalar $FU::REQ->{trace_sql}->@*)
+    },
+
+    fu => sub {
+        return () if !keys fu->%*;
+        # TODO: Contents of the 'fu' object
+        ('fu obj')
+    },
+
+    proc => sub {
+        table_ sub {
+            tr_ sub { td_ 'PID'; td_ $$ };
+            tr_ sub { td_ 'NAME'; td_ $0 };
+            tr_ sub { td_ 'ARGV'; td_ join ' ', @ARGV };
+            tr_ sub { td_ 'USER'; td_ "$< / $>" };
+            tr_ sub { td_ 'GROUP'; td_ "$( / $)" };
+            tr_ sub { td_ 'OS'; td_ $^O };
+            tr_ sub { td_ 'Perl'; td_ $^V };
+            tr_ sub { td_ 'Up for'; td_ sprintf '%.3f seconds', time - $^T };
+        };
+        ('Process')
+    },
+
+    env => sub {
+        table_ sub {
+            tr_ sub {
+                td_ $_;
+                td_ $ENV{$_};
+            } for sort keys %ENV;
+        };
+        ('Environment', scalar keys %ENV)
+    },
+
+    inc => sub {
+        table_ sub {
+            tr_ sub {
+                td_ $_;
+                td_ $INC{$_};
+            } for sort keys %INC;
+        };
+        ('Included files', scalar keys %INC)
+    },
+
+    han => sub {
+        my $cnt = 0;
+        my sub tbl_($title, $lst) {
+            return if !@$lst;
+            $cnt += @$lst;
+            h2_ $title;
+            table_ sub {
+                tr_ sub {
+                    td_ clean_re $_->[0];
+                    td_ sub { loc_ $_->[1] };
+                } for @$lst;
+            };
+        }
+        for my $meth (qw/GET POST DELETE OPTIONS PUT PATCH QUERY/) {
+            my($path, $re) = ($FU::path_routes{$meth}, $FU::re_routes{$meth});
+            my @lst = (
+                (map [$_, $path->{$_}[1]], $path ? sort keys %$path : ()),
+                (map [$_->[0], $_->[2]], $re ? @$re : ())
+            );
+            tbl_ $meth, \@lst;
+        }
+        tbl_ before_request => [ map [$_, $FU::before_request[$_][1]], 0..$#FU::before_request ];
+        tbl_ after_request => [ map [$_, $FU::after_request[$_][1]], 0..$#FU::after_request ];
+        ('Handlers', $cnt)
+    },
+
+    pgst => sub {
+        return () if !$FU::DB;
+        my $lst = eval { $FU::DB->q(
+            'SELECT generic_plans + custom_plans, statement FROM pg_prepared_statements ORDER BY generic_plans + custom_plans DESC, statement'
+        )->cache(0)->alla } || do { warn "Unable to collect prepared statement list: $@"; return () };
+        return () if !@$lst;
+        table_ sub {
+            thead_ sub { tr_ sub {
+                td_ 'Num';
+                td_ 'Query';
+            } };
+            tr_ sub {
+                td_ $_->[0];
+                td_ class => 'code', sub { fmtpre_ $_->[1] };
+            } for @$lst;
+        };
+        ('Prepared statements', scalar @$lst)
+    },
+);
+
+
+sub collect {
+    my @t;
+    for my ($id, $sub) (@tabs) {
+        my($title, $num);
+        my $html = fragment { ($title, $num) = $sub->() };
+        push @t, { id => $id, title => $title, num => $num, html => $html } if $title;
+    }
+    \@t
+}
+
+
+sub framework_($data) {
+    html_ sub {
+        head_ sub {
+            title_ 'FU Debugging Interface';
+            meta_ name => 'viewport', content => 'width=device-width, initial-scale=1.0, user-scalable=yes';
+            style_ type => 'text/css', <<~_;
+              html { box-sizing: border-box; color: #000; background: #fff }
+              *, *:before, *:after { box-sizing: inherit }
+              * { margin: 0; padding: 0; font: inherit; color: inherit }
+
+              body { display: grid; grid: 45px 400px / 220px auto; }
+              header { grid-column: 1 / 3; grid-row: 1 / 2 }
+              nav { grid-column: 1 / 2; grid-row: 2 / 3 }
+              main { grid-column: 2 / 3; grid-row: 2 / 3 }
+
+              header, nav { background: #eee }
+              main { border-top: 2px solid #009; border-left: 2px solid #009 }
+              nav { border-bottom: 2px solid #009 }
+
+              header { display: flex; justify-content: space-between; padding: 10px }
+              header h1 { font-size: 20px; font-weight: bold }
+              header menu { list-style-type: none; display: flex; gap: 15px }
+
+              body > input { display: none }
+              nav { padding-top: 20px }
+              nav menu { list-style-type: none }
+              nav label { display: block; width: 100%; padding: 2px 10px; cursor: pointer; white-space: nowrap }
+              nav label:hover { background-color: #fff }
+              nav label span { float: right; font-size: 80% }
+
+              main { padding: 10px 20px }
+              main h2 { margin: 30px 0 5px -10px; font-size: 20px; font-weight: bold }
+              main h2:first-child { margin-top: 0 }
+
+              p, pre, table { margin: 5px 0 }
+              pre, .code { font-family: monospace; white-space: pre }
+              table { border-collapse: collapse }
+              td { padding: 1px 10px 1px 0; font-size: 12px; vertical-align: top }
+              tr:hover { background-color: #eee }
+              thead { font-weight: bold }
+              .num { text-align: right; white-space: nowrap }
+            _
+            style_ type => 'text/css', join "\n", map +(
+                "#tab_$_:checked ~ nav menu li label[for=tab_$_] { background-color: #fff }",
+                "#tab_$_:not(:checked) ~ main #tabc_$_ { display: none }",
+            ), map $_->{id}, @$data;
+        };
+        body_ sub {
+            header_ sub {
+                h1_ 'FU Debugging Interface';
+                menu_ sub {
+                    li_ sub { a_ href => '?last', 'Last' };
+                    li_ sub { a_ href => '?cur', 'Current' };
+                    li_ sub { a_ href => '?', 'Listing' };
+                };
+            };
+            input_ type => 'radio', name => 'tab', id => "tab_$_->{id}", checked => $_ eq $data->[0] ? 'checked' : undef for @$data;
+            nav_ sub {
+                menu_ sub {
+                    li_ sub {
+                        label_ for => "tab_$_->{id}", sub {
+                            txt_ $_->{title};
+                            span_ $_->{num} if defined $_->{num};
+                        }
+                    } for @$data;
+                };
+            } if @$data;
+            main_ sub {
+                div_ id => "tabc_$_->{id}", sub {
+                    h2_ $_->{title};
+                    lit_ $_->{html};
+                } for @$data;
+            };
+        };
+    };
+}
+
+sub listing {
+    opendir my $dh, $FU::debug_info->{storage} or do {
+        warn "Error opening '$FU::debug_info->{storage}': $!\n";
+        return;
+    };
+    my @f;
+    /^fu-([0-9a-f]{22})\.txt$/ && push @f, $1 while (readdir $dh);
+    return [sort @f];
+}
+
+sub listing_ {
+    my $lst = listing;
+    return p_ 'Request logging disabled.' if !$FU::debug_info->{storage} || !$FU::debug_info->{history};
+    return p_ 'No requests logged.' if !@$lst;
+    table_ sub {
+        tr_ sub {
+            open my $fh, '<:utf8', "$FU::debug_info->{storage}/fu-$_.txt" or return;
+            my($ts, $time, $status, $method, $uri) = split / /, scalar <$fh>, 5;
+            td_ sub { a_ href => "?$_", $_ };
+            td_ class => 'num', fmtts $ts;
+            td_ class => 'num', sprintf '%.0f ms', $time*1000;
+            td_ class => 'num', $status;
+            td_ $method;
+            td_ $uri;
+        } for reverse @$lst;
+    }
+}
+
+sub load($id) {
+    open my $fn, '<', "$FU::debug_info->{storage}/fu-$id.txt" or fu->error(404);
+    scalar <$fn>;
+    local $/=undef;
+    fu->set_body(scalar <$fn>);
+}
+
+sub render {
+    my $q = fu->query;
+    if (!$q) {
+        fu->set_body(framework_ [{id => 'lst', title => 'Recent Requests', html => fragment \&listing_ }]);
+    } elsif ($q eq 'cur') {
+        fu->set_body(framework_ collect);
+    } elsif ($q eq 'last') {
+        my $lst = listing;
+        fu->error(404) if !@$lst;
+        load $lst->[$#$lst];
+    } elsif ($FU::debug_info->{storage} && $q =~ /^[0-9a-f]{22}$/) {
+        load $q;
+    } else {
+        fu->error(404);
+    }
+}
+
+sub save {
+    my $files = listing;
+    unlink sprintf '%s/fu-%s.txt', $FU::debug_info->{storage}, shift @$files while @$files >= $FU::debug_info->{history};
+
+    delete $FU::REQ->{txn};
+
+    my $fn = "$FU::debug_info->{storage}/fu-$FU::REQ->{trace_id}.txt";
+    open my $fh, '>', $fn or do {
+        warn "Error opening '$fn': $!\n";
+        return;
+    };
+    my $line = sprintf "%d %f %s %s %s\n",
+        time, time - $FU::REQ->{trace_start}, $FU::REQ->{status},
+        fu->method, fu->path.(fu->query?'?'.fu->query:'');
+    utf8::encode($line);
+    print $fh $line;
+    print $fh framework_ collect;
+}
+
+1;