diff --git a/docs/source/changing-output.rst b/docs/source/changing-output.rst
new file mode 100644
index 0000000..3a69b76
--- /dev/null
+++ b/docs/source/changing-output.rst
@@ -0,0 +1,157 @@
+.. Copyright 2023-2026 Tom Meltzer. See the top-level COPYRIGHT file for
+   details.
+
+.. _changing_output:
+
+Changing the Output Mode
+========================
+
+When debugging across multiple ranks, ``mdb`` can produce a large amount of output. By default,
+each rank's output is displayed independently, separated by divider lines. This is called *separate*
+mode and is the default behaviour.
+
+When the output from all ranks is identical, this can be redundant. The *combined* output mode
+reduces this verbosity by merging common lines across ranks into a single block, prefixing them
+with a combined rank range.
+
+This tutorial demonstrates the difference between the two modes using the ``set output`` command.
+
+The Example Program
+-------------------
+
+We will use the ``simple-mpi.exe`` binary from the ``examples/`` directory (see :ref:`quick_start`
+for details on compiling it).
+
+Launching the Debug Target
+--------------------------
+
+First, launch the program with two processes::
+
+    $ mdb launch -n 2 -t ./simple-mpi.exe
+
+This starts the debug server and waits for an ``mdb attach`` connection.
+
+Separate Output Mode (Default)
+------------------------------
+
+In separate mode, each rank's output is printed independently. The output from each rank is
+prefixed with its rank ID (e.g., ``0:`` or ``1:``) and blocks for different ranks are separated by
+a line of asterisks (``****``).
+
+Create a script file ``script-separate.mdb``::
+
+    set output separate
+    broadcast start
+    info proc
+    break 15
+    continue
+    list
+    continue
+    quit
+    quit
+
+Run it with::
+
+    $ mdb attach -x script-separate.mdb > separate.out
+
+Here is a shortened excerpt of the output from the ``info proc`` and ``list`` commands:
+
+**``info proc`` in separate mode:**
+
+.. code-block:: console
+
+   0: process 44900
+   0: cmdline = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
+   0: cwd = '/home/melt/sync/cambridge/projects/side/mdb'
+   0: exe = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
+   ************************************************************************
+   1: process 44903
+   1: cmdline = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
+   1: cwd = '/home/melt/sync/cambridge/projects/side/mdb'
+   1: exe = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
+
+Each rank's output appears in its own block, prefixed with just the rank number, separated by
+``****`` dividers.
+
+**``list`` in separate mode:**
+
+.. code-block:: console
+
+   0: 10
+   0: 11   call mpi_init(ierror)
+   0: 12   call mpi_comm_size(mpi_comm_world, size_of_cluster, ierror)
+   0: 13   call mpi_comm_rank(mpi_comm_world, process_rank, ierror)
+   0: 14
+   0: 15   var = 10.*process_rank
+   0: 16
+   0: 17   if (process_rank == 0) then
+   0: 18     print *, 'process 0 sleeping for 3s...'
+   0: 19     do i = 1, 3
+   ************************************************************************
+   1: 10
+   1: 11   call mpi_init(ierror)
+   1: 12   call mpi_comm_size(mpi_comm_world, size_of_cluster, ierror)
+   1: 13   call mpi_comm_rank(mpi_comm_world, process_rank, ierror)
+   1: 14
+   1: 15   var = 10.*process_rank
+   1: 16
+   1: 17   if (process_rank == 0) then
+   1: 18     print *, 'process 0 sleeping for 3s...'
+   1: 19     do i = 1, 3
+
+Since the source code is identical across ranks, the ``list`` output is duplicated for each rank.
+
+Combined Output Mode
+--------------------
+
+In combined mode, lines that are identical across all ranks are merged and prefixed with a combined
+rank range (e.g., ``0-1:``). Lines that differ between ranks are still shown per-rank with a single
+rank prefix (e.g., ``  0:``). No ``****`` dividers are used.
+
+Create a script file ``script-combined.mdb``::
+
+    set output combined
+    broadcast start
+    info proc
+    break 15
+    continue
+    list
+    continue
+    quit
+    quit
+
+Run it with::
+
+    $ mdb attach -x script-combined.mdb > combined.out
+
+**``info proc`` in combined mode:**
+
+.. code-block:: console
+
+     0: process 45004
+   0-1: cmdline = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
+   0-1: cwd = '/home/melt/sync/cambridge/projects/side/mdb'
+   0-1: exe = '/home/melt/sync/cambridge/projects/side/mdb/examples/simple-mpi.exe'
+     1: process 45005
+
+Notice how the three identical lines (``cmdline``, ``cwd``, ``exe``) are merged under a single
+``0-1:`` prefix. The ``process`` line differs between ranks (different PIDs) so it is shown
+individually for each rank.
+
+**``list`` in combined mode:**
+
+.. code-block:: console
+
+   0-1: 10
+   0-1: 11   call mpi_init(ierror)
+   0-1: 12   call mpi_comm_size(mpi_comm_world, size_of_cluster, ierror)
+   0-1: 13   call mpi_comm_rank(mpi_comm_world, process_rank, ierror)
+   0-1: 14
+   0-1: 15   var = 10.*process_rank
+   0-1: 16
+   0-1: 17   if (process_rank == 0) then
+   0-1: 18     print *, 'process 0 sleeping for 3s...'
+   0-1: 19     do i = 1, 3
+
+Since all ranks share the same source code, every line is identical and merged into a single
+compact block. This avoids the duplication seen in separate mode.
diff --git a/docs/source/index.rst b/docs/source/index.rst
index eccdf0e..93db5f0 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -19,6 +19,7 @@ serial debugger backends e.g.,  `gdb <https://sourceware.org/gdb>`_ and `lldb
 
    Installation <installation>
    Quick Start <quick-start>
+   Changing the Output Mode <changing-output>
    Debugging AMD GPU Kernels <gpu-amd>
 
 .. toctree::
diff --git a/pyproject.toml b/pyproject.toml
index be47452..3dce575 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "mdb_debugger"
-version = "1.0.6"
+version = "1.0.7"
 dependencies = [
     "click==8.1.7",
     "matplotlib==3.8.3",
diff --git a/src/mdb/mdb_shell.py b/src/mdb/mdb_shell.py
index 657c2b2..ff26571 100644
--- a/src/mdb/mdb_shell.py
+++ b/src/mdb/mdb_shell.py
@@ -24,6 +24,7 @@
     parse_ranks,
     pretty_print_response,
     sort_debug_response,
+    reduce_response,
 )
 
 if TYPE_CHECKING:
@@ -55,6 +56,7 @@ def __init__(self, shell_opts: ShellOpts, client: Client) -> None:
         self.exchange_select = parse_ranks(self.exchange_select_str)
         self.select_str = self.exchange_select_str
         self.select = self.exchange_select
+        self.output_mode = "separate"  # 'separate' or 'combined'
         backend_name = shell_opts["backend_name"].lower()
         if backend_name in backends:
             self.backend = backends[backend_name]()
@@ -214,7 +216,10 @@ def ask_remain_calm(signame: str) -> None:
 
         if command_response.msg_type == "exchange_command_response":
             response = sort_debug_response(command_response.data["results"])
-            pretty_print_response(response)
+            if self.output_mode == "combined":
+                reduce_response(response)
+            else:
+                pretty_print_response(response)
         else:
             print("Received unexpected message type: %s", command_response.msg_type)
         return
@@ -250,6 +255,44 @@ def do_shell(self, line: str) -> None:
         run(split(line))
         return
 
+    def do_set(self, line: str) -> None:
+        """
+        Description:
+        Set mdb options.
+
+        Example:
+        Switch output format between separate and combined mode:
+
+            (mdb) set output combined
+            (mdb) set output separate
+
+        - separate shows all output for each rank separate by ***'s
+        - combined reduces common output across the ranks
+
+        Show current settings:
+
+            (mdb) set
+        """
+        if not line:
+            print(f"output: {self.output_mode}")
+            return
+
+        parts = line.split()
+        if len(parts) < 2:
+            print("Usage: set output [separate|combined]")
+            return
+
+        if parts[0].lower() == "output":
+            mode = parts[1].lower()
+            if mode in ("separate", "combined"):
+                self.output_mode = mode
+            else:
+                print(
+                    f"Error: unknown output mode '{mode}'. Use 'separate' or 'combined'."
+                )
+        else:
+            print(f"Error: unknown option '{parts[0]}'. Use 'output'.")
+
     def do_select(self, line: str) -> None:
         """
         Description:
diff --git a/src/mdb/utils.py b/src/mdb/utils.py
index 3ac7f88..de9f34f 100644
--- a/src/mdb/utils.py
+++ b/src/mdb/utils.py
@@ -4,6 +4,7 @@
 import re
 from os.path import expanduser
 from typing import TYPE_CHECKING
+from collections import defaultdict
 
 if TYPE_CHECKING:
     from .backend import DebugBackend
@@ -21,6 +22,29 @@ def sort_debug_response(results: dict[int, str]) -> dict[int, str]:
     return dict(sorted(results.items()))
 
 
+def collapse_ranges(values: list[int]) -> str:
+    """Collapse a list of integers into minimal range notation.
+
+    e.g. [1, 2, 3, 6, 7, 10, 11, 12] -> '1-3,6-7,10-12'
+    """
+    sorted_vals = sorted(set(values))
+    if not sorted_vals:
+        return ""
+
+    ranges = []
+    start = end = sorted_vals[0]
+
+    for n in sorted_vals[1:]:
+        if n == end + 1:
+            end = n
+        else:
+            ranges.append(f"{start}-{end}" if start != end else str(start))
+            start = end = n
+
+    ranges.append(f"{start}-{end}" if start != end else str(start))
+    return ",".join(ranges)
+
+
 def pretty_print_response(response: dict[int, str]) -> None:
     lines = []
     for rank, result in response.items():
@@ -30,6 +54,53 @@ def pretty_print_response(response: dict[int, str]) -> None:
     print(combined_output)
 
 
+def reduce_response(response: dict[int, str]) -> None:
+    """Reduce debug output by deduplicating common lines across ranks.
+
+    Parses each rank's output, groups identical lines together, and prints
+    each unique line prefixed by the collapsed set of ranks that produced it.
+    Lines from only one rank appear normally; shared lines are printed once
+    with all contributing ranks.
+
+    Example output before and after:
+
+        Before (raw output per rank):
+
+            0:      process 45402
+            0:      cmdline = '/mdb/examples/simple-mpi.exe'
+            0:      cwd = '/mdb'
+            0:      exe = '/mdb/examples/simple-mpi.exe'
+            ************************************************************************
+            1:      process 45403
+            1:      cmdline = '/mdb/examples/simple-mpi.exe'
+            1:      cwd = '/mdb'
+            1:      exe = '/mdb/examples/simple-mpi.exe'
+
+        After (deduplicated, ranks collapsed):
+
+              0: process 45402
+            0-1: cmdline = '/mdb/examples/simple-mpi.exe'
+            0-1: cwd = '/mdb'
+            0-1: exe = '/mdb/examples/simple-mpi.exe'
+              1: process 45403
+
+    Args:
+        response: dict mapping process rank (int) to its output string.
+    """
+    reduced = defaultdict(list)
+    for rank, result in response.items():
+        if result:
+            for line in result.split("\r\n")[1:-1]:
+                reduced[line].append(rank)
+
+    reduced = {k: collapse_ranges(v) for k, v in reduced.items()}
+
+    max_len = max([len(v) for v in reduced.values()], default=0)
+    for line, ranks_str in reduced.items():
+        padded = ranks_str.rjust(max_len)
+        print(f"{padded}: {line}")
+
+
 def extract_float(line: str, backend: "DebugBackend") -> float:
     float_regex = backend.float_regex
     line = strip_control_characters(line)
@@ -49,9 +120,7 @@ def extract_float(line: str, backend: "DebugBackend") -> float:
 
 
 def prepend_ranks(rank: int, result: str) -> str:
-    return "".join(
-        [f"{rank}:\t" + line + "\r\n" for line in result.split("\r\n")[1:-1]]
-    )
+    return "".join([f"{rank}: " + line + "\r\n" for line in result.split("\r\n")[1:-1]])
 
 
 def strip_bracketted_paste(text: str) -> str:
diff --git a/tests/output/answer-gdb.stdout b/tests/output/answer-gdb.stdout
index ac72227..22a9a2f 100644
--- a/tests/output/answer-gdb.stdout
+++ b/tests/output/answer-gdb.stdout
@@ -1,57 +1,57 @@
 hello
-0:cmdline = simple-mpi.exe
-0:cwd = [mdb root]
-0:exe = simple-mpi.exe
+0: cmdline = simple-mpi.exe
+0: cwd = [mdb root]
+0: exe = simple-mpi.exe
 ************************************************************************
-1:cmdline = simple-mpi.exe
-1:cwd = [mdb root]
-1:exe = simple-mpi.exe
-0:Breakpoint 2 at [hex address]: file simple-mpi.f90, line 15.
+1: cmdline = simple-mpi.exe
+1: cwd = [mdb root]
+1: exe = simple-mpi.exe
+0: Breakpoint 2 at [hex address]: file simple-mpi.f90, line 15.
 ************************************************************************
-1:Breakpoint 2 at [hex address]: file simple-mpi.f90, line 15.
-0:Breakpoint 3 at [hex address]: file simple-mpi.f90, line 17.
+1: Breakpoint 2 at [hex address]: file simple-mpi.f90, line 15.
+0: Breakpoint 3 at [hex address]: file simple-mpi.f90, line 17.
 ************************************************************************
-1:Breakpoint 3 at [hex address]: file simple-mpi.f90, line 17.
-0:Continuing.
+1: Breakpoint 3 at [hex address]: file simple-mpi.f90, line 17.
+0: Continuing.
 0:
-0:Thread 1 "simple-mpi.exe" hit Breakpoint 2, simple () at simple-mpi.f90:15
-0:15  var = 10.*process_rank
+0: Thread 1 "simple-mpi.exe" hit Breakpoint 2, simple () at simple-mpi.f90:15
+0: 15  var = 10.*process_rank
 ************************************************************************
-1:Continuing.
+1: Continuing.
 1:
-1:Thread 1 "simple-mpi.exe" hit Breakpoint 2, simple () at simple-mpi.f90:15
-1:15  var = 10.*process_rank
-0:Continuing.
+1: Thread 1 "simple-mpi.exe" hit Breakpoint 2, simple () at simple-mpi.f90:15
+1: 15  var = 10.*process_rank
+0: Continuing.
 0:
-0:Thread 1 "simple-mpi.exe" hit Breakpoint 3, simple () at simple-mpi.f90:17
-0:17  if (process_rank == 0) then
-0:#0  simple () at simple-mpi.f90:17
+0: Thread 1 "simple-mpi.exe" hit Breakpoint 3, simple () at simple-mpi.f90:17
+0: 17  if (process_rank == 0) then
+0: #0  simple () at simple-mpi.f90:17
 ************************************************************************
-1:#0  simple () at simple-mpi.f90:15
+1: #0  simple () at simple-mpi.f90:15
 unrecognized command [made-up-command]. Type help to find out list of possible commands.
 Error: user specified option [select] must be subset of available ranks (check mdb launch command).
 select = [10] but available ranks are [0-1].
-1:Continuing.
+1: Continuing.
 1:
-1:Thread 1 "simple-mpi.exe" hit Breakpoint 3, simple () at simple-mpi.f90:17
-1:17  if (process_rank == 0) then
+1: Thread 1 "simple-mpi.exe" hit Breakpoint 3, simple () at simple-mpi.f90:17
+1: 17  if (process_rank == 0) then
 File [deliberately-missing-file.mdb] not found. Please check the file exists and try again.
-0:25
-************************************************************************
-1:25
-0:Continuing.
-0:           1 s...
-0:           2 s...
-0:           3 s...
-0: in level 1
-0: in level 2
-0: internal process:            0 of            2
-0: var =    0.00000000
-************************************************************************
-1:Continuing.
-1: in level 1
-1: in level 2
-1: internal process:            1 of            2
-1: var =    10.0000000
+0: 25
+************************************************************************
+1: 25
+0: Continuing.
+0:            1 s...
+0:            2 s...
+0:            3 s...
+0:  in level 1
+0:  in level 2
+0:  internal process:            0 of            2
+0:  var =    0.00000000
+************************************************************************
+1: Continuing.
+1:  in level 1
+1:  in level 2
+1:  internal process:            1 of            2
+1:  var =    10.0000000
 ************************************************************************
 exiting mdb...
\ No newline at end of file
diff --git a/tests/output/answer-lldb.stdout b/tests/output/answer-lldb.stdout
index ca6d5fb..292e036 100644
--- a/tests/output/answer-lldb.stdout
+++ b/tests/output/answer-lldb.stdout
@@ -1,89 +1,89 @@
 hello
-0:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 1.1
-0:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:24:7
-0:   21    int process_rank, size_of_cluster;
-0:   22    float var;
-0:   23
-0:-> 24    var = 0.;
-0:          ^
-0:   25
-0:   26    MPI_Init(NULL, NULL);
-0:   27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
+0: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 1.1
+0:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:24:7
+0:    21    int process_rank, size_of_cluster;
+0:    22    float var;
+0:    23
+0: -> 24    var = 0.;
+0:           ^
+0:    25
+0:    26    MPI_Init(NULL, NULL);
+0:    27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
 ************************************************************************
-1:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 1.1
-1:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:24:7
-1:   21    int process_rank, size_of_cluster;
-1:   22    float var;
-1:   23
-1:-> 24    var = 0.;
-1:          ^
-1:   25
-1:   26    MPI_Init(NULL, NULL);
-1:   27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
-0:Breakpoint 2: where = simple-mpi-cpp.exe`main + 95 at simple-mpi-cpp.cpp:30:12, address = [hex address]
+1: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 1.1
+1:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:24:7
+1:    21    int process_rank, size_of_cluster;
+1:    22    float var;
+1:    23
+1: -> 24    var = 0.;
+1:           ^
+1:    25
+1:    26    MPI_Init(NULL, NULL);
+1:    27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
+0: Breakpoint 2: where = simple-mpi-cpp.exe`main + 95 at simple-mpi-cpp.cpp:30:12, address = [hex address]
 ************************************************************************
-1:Breakpoint 2: where = simple-mpi-cpp.exe`main + 95 at simple-mpi-cpp.cpp:30:12, address = [hex address]
-0:Breakpoint 3: where = simple-mpi-cpp.exe`main + 127 at simple-mpi-cpp.cpp:32:20, address = [hex address]
+1: Breakpoint 2: where = simple-mpi-cpp.exe`main + 95 at simple-mpi-cpp.cpp:30:12, address = [hex address]
+0: Breakpoint 3: where = simple-mpi-cpp.exe`main + 127 at simple-mpi-cpp.cpp:32:20, address = [hex address]
 ************************************************************************
-1:Breakpoint 3: where = simple-mpi-cpp.exe`main + 127 at simple-mpi-cpp.cpp:32:20, address = [hex address]
-0:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 2.1
-0:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:30:12
-0:   27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
-0:   28    MPI_Comm_rank(MPI_COMM_WORLD, &process_rank);
-0:   29
-0:-> 30    var = 10.*process_rank;
-0:               ^
-0:   31
-0:   32    if (process_rank == 0){
+1: Breakpoint 3: where = simple-mpi-cpp.exe`main + 127 at simple-mpi-cpp.cpp:32:20, address = [hex address]
+0: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 2.1
+0:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:30:12
+0:    27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
+0:    28    MPI_Comm_rank(MPI_COMM_WORLD, &process_rank);
+0:    29
+0: -> 30    var = 10.*process_rank;
+0:                ^
+0:    31
+0:    32    if (process_rank == 0){
 ************************************************************************
-1:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 2.1
-1:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:30:12
-1:   27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
-1:   28    MPI_Comm_rank(MPI_COMM_WORLD, &process_rank);
-1:   29
-1:-> 30    var = 10.*process_rank;
-1:               ^
-1:   31
-1:   32    if (process_rank == 0){
-0:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 3.1
-0:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:32:20
-0:   29
-0:   30    var = 10.*process_rank;
-0:   31
-0:-> 32    if (process_rank == 0){
-0:                       ^
-0:   34
-0:   35      for (int i = 0; i < 3; ++i) {
-0:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 3.1
-0:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:32:20
+1: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 2.1
+1:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:30:12
+1:    27    MPI_Comm_size(MPI_COMM_WORLD, &size_of_cluster);
+1:    28    MPI_Comm_rank(MPI_COMM_WORLD, &process_rank);
+1:    29
+1: -> 30    var = 10.*process_rank;
+1:                ^
+1:    31
+1:    32    if (process_rank == 0){
+0: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 3.1
+0:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:32:20
+0:    29
+0:    30    var = 10.*process_rank;
+0:    31
+0: -> 32    if (process_rank == 0){
+0:                        ^
+0:    34
+0:    35      for (int i = 0; i < 3; ++i) {
+0: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 3.1
+0:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:32:20
 ************************************************************************
-1:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 2.1
-1:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:30:12
+1: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 2.1
+1:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:30:12
 unrecognized command [made-up-command]. Type help to find out list of possible commands.
-1:* thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 3.1
-1:    frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:32:20
-1:   29
-1:   30    var = 10.*process_rank;
-1:   31
-1:-> 32    if (process_rank == 0){
-1:                       ^
-1:   34
-1:   35      for (int i = 0; i < 3; ++i) {
+1: * thread #1, name = 'simple-mpi-cpp.', stop reason = breakpoint 3.1
+1:     frame #0: [hex address] simple-mpi-cpp.exe`main at simple-mpi-cpp.cpp:32:20
+1:    29
+1:    30    var = 10.*process_rank;
+1:    31
+1: -> 32    if (process_rank == 0){
+1:                        ^
+1:    34
+1:    35      for (int i = 0; i < 3; ++i) {
 File [deliberately-missing-file.mdb] not found. Please check the file exists and try again.
-0:(int) 25
+0: (int) 25
 ************************************************************************
-1:(int) 25
-0:0 s...
-0:1 s...
-0:2 s...
-0:in level 1
-0:in level 2
-0:internal process: 0 of 2
-0:var = 0
+1: (int) 25
+0: 0 s...
+0: 1 s...
+0: 2 s...
+0: in level 1
+0: in level 2
+0: internal process: 0 of 2
+0: var = 0
 ************************************************************************
-1:in level 1
-1:in level 2
-1:internal process: 1 of 2
-1:var = 10
+1: in level 1
+1: in level 2
+1: internal process: 1 of 2
+1: var = 10
 ************************************************************************
 exiting mdb...
\ No newline at end of file
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 8dc9cac..a9401f0 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,7 +1,16 @@
 # Copyright 2023-2026 Tom Meltzer. See the top-level COPYRIGHT file for
 # details.
 
-from mdb.utils import parse_ranks, strip_bracketted_paste, strip_control_characters
+import pytest
+
+from mdb.utils import (
+    collapse_ranges,
+    parse_ranks,
+    pretty_print_response,
+    reduce_response,
+    strip_bracketted_paste,
+    strip_control_characters,
+)
 
 
 def test_parse_ranks() -> None:
@@ -18,6 +27,76 @@ def test_parse_ranks() -> None:
     assert ranks == [2, 3, 4]
 
 
+def test_collapse_ranges() -> None:
+    # consecutive integers collapse to a range
+    assert collapse_ranges([1, 2, 3, 4]) == "1-4"
+
+    # single integer
+    assert collapse_ranges([5]) == "5"
+
+    # multiple disjoint ranges
+    assert collapse_ranges([1, 2, 3, 6, 7, 10, 11, 12]) == "1-3,6-7,10-12"
+
+    # unsorted input is sorted internally
+    assert collapse_ranges([12, 10, 11, 7, 6, 3, 2, 1]) == "1-3,6-7,10-12"
+
+    # duplicates are deduplicated
+    assert collapse_ranges([1, 1, 2, 2, 3]) == "1-3"
+
+    # empty list returns empty string
+    assert collapse_ranges([]) == ""
+
+    # gaps between single values
+    assert collapse_ranges([1, 3, 5, 7]) == "1,3,5,7"
+
+    # mix of single values and ranges
+    assert collapse_ranges([1, 2, 5, 6, 7, 10]) == "1-2,5-7,10"
+
+
+def test_pretty_print_response(capsys: pytest.CaptureFixture[str]) -> None:
+    response = {
+        0: "command ran\r\nline 1\r\nline 2\r\n",
+        1: "command ran\r\nline 3\r\n",
+    }
+    pretty_print_response(response)
+    captured = capsys.readouterr()
+
+    assert "0: line 1" in captured.out
+    assert "0: line 2" in captured.out
+    assert "1: line 3" in captured.out
+    assert "*" * 72 in captured.out  # separator between ranks
+    # header lines are skipped
+    assert "command ran" not in captured.out
+
+
+def test_reduce_response(capsys: pytest.CaptureFixture[str]) -> None:
+    response = {
+        0: "header\r\ncommon line\r\nunique to 0\r\n",
+        1: "header\r\ncommon line\r\nunique to 1\r\n",
+        2: "header\r\ncommon line\r\n",
+    }
+    reduce_response(response)
+    captured = capsys.readouterr()
+
+    # shared line should be prefixed with collapsed rank range
+    assert "0-2: common line" in captured.out
+    # unique lines should show only their rank
+    assert "  0: unique to 0" in captured.out
+    assert "  1: unique to 1" in captured.out
+    # header line is skipped
+    assert "header" not in captured.out
+
+
+def test_reduce_response_single_rank(capsys: pytest.CaptureFixture[str]) -> None:
+    response = {
+        3: "header\r\nonly line\r\n",
+    }
+    reduce_response(response)
+    captured = capsys.readouterr()
+
+    assert "3: only line" in captured.out
+
+
 def test_strip_functions() -> None:
     text = "bt\r\n\x1b[?2004l\r#0  \x1b[33msimple\x1b[m () at \x1b[32msimple-mpi.f90\x1b[m:1\r\n\x1b[?2004h"
     text = strip_bracketted_paste(text)