From 4d7585a7111eb3515fc7772d9b33af1183ed3b65 Mon Sep 17 00:00:00 2001
From: Liam <liam@scalzulli.com>
Date: Fri, 9 Jan 2026 11:23:52 -0500
Subject: [PATCH 1/2] Add `--id` flag to `cargo report timings` and `cargo
 report rebuilds`

This adds the `--id` argument to the CLI definitions but does not yet
implement the functionality. The argument is currently accepted but
ignored; the commands still use the most recent session by default.

This commit also adds tests that verify the current behavior (using the
most recent session) in preparation for the next commit which implements
the `--id` functionality.
---
 src/bin/cargo/commands/report.rs              |  6 ++-
 tests/testsuite/cargo_report_rebuilds/mod.rs  | 44 +++++++++++++++++
 .../cargo_report_timings/help/stdout.term.svg | 32 +++++++------
 tests/testsuite/cargo_report_timings/mod.rs   | 47 +++++++++++++++++++
 4 files changed, 112 insertions(+), 17 deletions(-)

diff --git a/src/bin/cargo/commands/report.rs b/src/bin/cargo/commands/report.rs
index e31edb19133..499a208c905 100644
--- a/src/bin/cargo/commands/report.rs
+++ b/src/bin/cargo/commands/report.rs
@@ -31,7 +31,8 @@ pub fn cli() -> Command {
             subcommand("timings")
                 .about("Reports the build timings of previous sessions (unstable)")
                 .arg_manifest_path()
-                .arg(flag("open", "Opens the timing report in a browser")),
+                .arg(flag("open", "Opens the timing report in a browser"))
+                .arg(opt("id", "Session ID to report on").value_name("ID")),
         )
         .subcommand(
             subcommand("sessions")
@@ -47,7 +48,8 @@ pub fn cli() -> Command {
         .subcommand(
             subcommand("rebuilds")
                 .about("Reports rebuild reasons from previous sessions (unstable)")
-                .arg_manifest_path(),
+                .arg_manifest_path()
+                .arg(opt("id", "Session ID to report on").value_name("ID")),
         )
 }
 
diff --git a/tests/testsuite/cargo_report_rebuilds/mod.rs b/tests/testsuite/cargo_report_rebuilds/mod.rs
index 8aada08d77f..2c2c2e0c866 100644
--- a/tests/testsuite/cargo_report_rebuilds/mod.rs
+++ b/tests/testsuite/cargo_report_rebuilds/mod.rs
@@ -563,3 +563,47 @@ Root rebuilds:
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn uses_latest_session_by_default() {
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .file("src/lib.rs", "")
+        .build();
+
+    // First session: fresh build (1 new unit)
+    p.cargo("check -Zbuild-analysis")
+        .env("CARGO_BUILD_ANALYSIS_ENABLED", "true")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .run();
+
+    let _ = paths::log_file(0);
+
+    p.change_file("src/lib.rs", "// touched");
+
+    // Second session: rebuild (1 unit rebuilt)
+    p.cargo("check -Zbuild-analysis")
+        .env("CARGO_BUILD_ANALYSIS_ENABLED", "true")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .run();
+
+    let _ = paths::log_file(1);
+
+    // Without --id, should use the most recent (second) session which has a rebuild
+    p.cargo("report rebuilds -Zbuild-analysis")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .with_stderr_data(str![[r#"
+Session: [..]
+Status: 1 unit rebuilt, 0 cached, 0 new
+
+Rebuild impact:
+  root rebuilds: 1 unit
+  cascading:     0 units
+
+Root rebuilds:
+  0. foo@0.0.0 (check): file modified: src/lib.rs
+     impact: no cascading rebuilds
+
+"#]])
+        .run();
+}
diff --git a/tests/testsuite/cargo_report_timings/help/stdout.term.svg b/tests/testsuite/cargo_report_timings/help/stdout.term.svg
index 0fd20351a5d..00b692de7cd 100644
--- a/tests/testsuite/cargo_report_timings/help/stdout.term.svg
+++ b/tests/testsuite/cargo_report_timings/help/stdout.term.svg
@@ -1,4 +1,4 @@
-<svg width="827px" height="380px" xmlns="http://www.w3.org/2000/svg">
+<svg width="827px" height="398px" xmlns="http://www.w3.org/2000/svg">
   <style>
     .fg { fill: #AAAAAA }
     .bg { fill: #000000 }
@@ -32,33 +32,35 @@
 </tspan>
     <tspan x="10px" y="118px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--open</tspan><tspan>                     Opens the timing report in a browser</tspan>
 </tspan>
-    <tspan x="10px" y="136px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-v</tspan><tspan>, </tspan><tspan class="fg-bright-cyan bold">--verbose</tspan><tspan class="fg-cyan">...</tspan><tspan>               Use verbose output (-vv very verbose/build.rs output)</tspan>
+    <tspan x="10px" y="136px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--id</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;ID&gt;</tspan><tspan>                  Session ID to report on</tspan>
 </tspan>
-    <tspan x="10px" y="154px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-q</tspan><tspan>, </tspan><tspan class="fg-bright-cyan bold">--quiet</tspan><tspan>                    Do not print cargo log messages</tspan>
+    <tspan x="10px" y="154px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-v</tspan><tspan>, </tspan><tspan class="fg-bright-cyan bold">--verbose</tspan><tspan class="fg-cyan">...</tspan><tspan>               Use verbose output (-vv very verbose/build.rs output)</tspan>
 </tspan>
-    <tspan x="10px" y="172px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--color</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;WHEN&gt;</tspan><tspan>             Coloring [possible values: auto, always, never]</tspan>
+    <tspan x="10px" y="172px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-q</tspan><tspan>, </tspan><tspan class="fg-bright-cyan bold">--quiet</tspan><tspan>                    Do not print cargo log messages</tspan>
 </tspan>
-    <tspan x="10px" y="190px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--config</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;KEY=VALUE|PATH&gt;</tspan><tspan>  Override a configuration value</tspan>
+    <tspan x="10px" y="190px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--color</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;WHEN&gt;</tspan><tspan>             Coloring [possible values: auto, always, never]</tspan>
 </tspan>
-    <tspan x="10px" y="208px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-Z</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;FLAG&gt;</tspan><tspan>                      Unstable (nightly-only) flags to Cargo, see 'cargo -Z help' for</tspan>
+    <tspan x="10px" y="208px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--config</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;KEY=VALUE|PATH&gt;</tspan><tspan>  Override a configuration value</tspan>
 </tspan>
-    <tspan x="10px" y="226px"><tspan>                                 details</tspan>
+    <tspan x="10px" y="226px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-Z</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;FLAG&gt;</tspan><tspan>                      Unstable (nightly-only) flags to Cargo, see 'cargo -Z help' for</tspan>
 </tspan>
-    <tspan x="10px" y="244px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-h</tspan><tspan>, </tspan><tspan class="fg-bright-cyan bold">--help</tspan><tspan>                     Print help</tspan>
+    <tspan x="10px" y="244px"><tspan>                                 details</tspan>
 </tspan>
-    <tspan x="10px" y="262px">
+    <tspan x="10px" y="262px"><tspan>  </tspan><tspan class="fg-bright-cyan bold">-h</tspan><tspan>, </tspan><tspan class="fg-bright-cyan bold">--help</tspan><tspan>                     Print help</tspan>
 </tspan>
-    <tspan x="10px" y="280px"><tspan class="fg-bright-green bold">Manifest Options:</tspan>
+    <tspan x="10px" y="280px">
 </tspan>
-    <tspan x="10px" y="298px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--manifest-path</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;PATH&gt;</tspan><tspan>  Path to Cargo.toml</tspan>
+    <tspan x="10px" y="298px"><tspan class="fg-bright-green bold">Manifest Options:</tspan>
 </tspan>
-    <tspan x="10px" y="316px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--locked</tspan><tspan>                Assert that `Cargo.lock` will remain unchanged</tspan>
+    <tspan x="10px" y="316px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--manifest-path</tspan><tspan class="fg-cyan"> </tspan><tspan class="fg-cyan">&lt;PATH&gt;</tspan><tspan>  Path to Cargo.toml</tspan>
 </tspan>
-    <tspan x="10px" y="334px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--offline</tspan><tspan>               Run without accessing the network</tspan>
+    <tspan x="10px" y="334px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--locked</tspan><tspan>                Assert that `Cargo.lock` will remain unchanged</tspan>
 </tspan>
-    <tspan x="10px" y="352px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--frozen</tspan><tspan>                Equivalent to specifying both --locked and --offline</tspan>
+    <tspan x="10px" y="352px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--offline</tspan><tspan>               Run without accessing the network</tspan>
 </tspan>
-    <tspan x="10px" y="370px">
+    <tspan x="10px" y="370px"><tspan>      </tspan><tspan class="fg-bright-cyan bold">--frozen</tspan><tspan>                Equivalent to specifying both --locked and --offline</tspan>
+</tspan>
+    <tspan x="10px" y="388px">
 </tspan>
   </text>
 
diff --git a/tests/testsuite/cargo_report_timings/mod.rs b/tests/testsuite/cargo_report_timings/mod.rs
index 8c4661fdf17..03cd6941318 100644
--- a/tests/testsuite/cargo_report_timings/mod.rs
+++ b/tests/testsuite/cargo_report_timings/mod.rs
@@ -345,3 +345,50 @@ fn with_multiple_targets() {
 
     assert_eq!(p.glob("**/cargo-timing-*.html").count(), 1);
 }
+
+#[cargo_test]
+fn uses_latest_session_by_default() {
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -Zbuild-analysis")
+        .env("CARGO_BUILD_ANALYSIS_ENABLED", "true")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .run();
+
+    let first_log = paths::log_file(0);
+    let first_session_id = first_log.file_stem().unwrap().to_str().unwrap();
+
+    p.change_file("src/lib.rs", "pub fn foo() {}");
+    p.cargo("check -Zbuild-analysis")
+        .env("CARGO_BUILD_ANALYSIS_ENABLED", "true")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .run();
+
+    let second_log = paths::log_file(1);
+    let second_session_id = second_log.file_stem().unwrap().to_str().unwrap();
+
+    // Without --id, should use the most recent (second) session
+    p.cargo("report timings -Zbuild-analysis")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .with_stderr_data(str![[r#"
+      Timing report saved to [ROOT]/foo/target/cargo-timings/cargo-timing-[..]T[..]Z-[..].html
+
+"#]])
+        .run();
+
+    let timing_files: Vec<_> = p.glob("**/cargo-timing-*.html").collect();
+    assert_eq!(timing_files.len(), 1);
+    let timing_file = timing_files[0].as_ref().unwrap();
+    let filename = timing_file.file_name().unwrap().to_str().unwrap();
+    assert!(
+        filename.contains(second_session_id),
+        "Expected timing file to contain second session ID {second_session_id}, got {filename}"
+    );
+    assert!(
+        !filename.contains(first_session_id),
+        "Should not contain first session ID {first_session_id}, got {filename}"
+    );
+}

From de85543b5f560d4dc0de203b1b620da9dbe67374 Mon Sep 17 00:00:00 2001
From: Liam <liam@scalzulli.com>
Date: Fri, 9 Jan 2026 11:28:47 -0500
Subject: [PATCH 2/2] Implement `--id` flag for `cargo report timings` and
 `cargo report rebuilds`

This implements the `--id` flag that was added in the previous commit.
When specified, the commands will report on the session with the given
ID instead of the most recent session.

The implementation adds a `find_log_file` helper that searches for a
specific session ID when provided, or falls back to returning the most
recent session when no ID is specified.

Error handling is updated to provide helpful messages:
- When a specific ID is not found, users are directed to
  `cargo report sessions` to list available sessions
- Invalid ID formats are rejected with a clear error message
---
 src/bin/cargo/commands/report.rs             | 19 +++++-
 src/cargo/ops/cargo_report/rebuilds.rs       | 23 +++++--
 src/cargo/ops/cargo_report/timings.rs        | 18 ++++--
 src/cargo/ops/cargo_report/util.rs           | 18 ++++++
 tests/testsuite/cargo_report_rebuilds/mod.rs | 61 +++++++++++++++----
 tests/testsuite/cargo_report_timings/mod.rs  | 63 ++++++++++++++++----
 6 files changed, 167 insertions(+), 35 deletions(-)

diff --git a/src/bin/cargo/commands/report.rs b/src/bin/cargo/commands/report.rs
index 499a208c905..c2d03e86058 100644
--- a/src/bin/cargo/commands/report.rs
+++ b/src/bin/cargo/commands/report.rs
@@ -122,8 +122,16 @@ fn timings_opts<'a>(
     args: &ArgMatches,
 ) -> CargoResult<ops::ReportTimingsOptions<'a>> {
     let open_result = args.get_flag("open");
+    let id = args
+        .get_one::<String>("id")
+        .map(|s| s.parse())
+        .transpose()?;
 
-    Ok(ops::ReportTimingsOptions { open_result, gctx })
+    Ok(ops::ReportTimingsOptions {
+        open_result,
+        gctx,
+        id,
+    })
 }
 
 fn sessions_opts(args: &ArgMatches) -> CargoResult<ops::ReportSessionsOptions> {
@@ -133,6 +141,11 @@ fn sessions_opts(args: &ArgMatches) -> CargoResult<ops::ReportSessionsOptions> {
     Ok(ops::ReportSessionsOptions { limit })
 }
 
-fn rebuilds_opts(_args: &ArgMatches) -> CargoResult<ops::ReportRebuildsOptions> {
-    Ok(ops::ReportRebuildsOptions {})
+fn rebuilds_opts(args: &ArgMatches) -> CargoResult<ops::ReportRebuildsOptions> {
+    let id = args
+        .get_one::<String>("id")
+        .map(|s| s.parse())
+        .transpose()?;
+
+    Ok(ops::ReportRebuildsOptions { id })
 }
diff --git a/src/cargo/ops/cargo_report/rebuilds.rs b/src/cargo/ops/cargo_report/rebuilds.rs
index cb95a1c714d..8d0495d1d2c 100644
--- a/src/cargo/ops/cargo_report/rebuilds.rs
+++ b/src/cargo/ops/cargo_report/rebuilds.rs
@@ -20,7 +20,7 @@ use crate::core::compiler::UnitIndex;
 use crate::core::compiler::fingerprint::DirtyReason;
 use crate::core::compiler::fingerprint::FsStatus;
 use crate::core::compiler::fingerprint::StaleItem;
-use crate::ops::cargo_report::util::list_log_files;
+use crate::ops::cargo_report::util::find_log_file;
 use crate::ops::cargo_report::util::unit_target_description;
 use crate::util::log_message::FingerprintStatus;
 use crate::util::log_message::LogMessage;
@@ -30,21 +30,32 @@ use crate::util::style;
 
 const DEFAULT_DISPLAY_LIMIT: usize = 5;
 
-pub struct ReportRebuildsOptions {}
+pub struct ReportRebuildsOptions {
+    pub id: Option<RunId>,
+}
 
 pub fn report_rebuilds(
     gctx: &GlobalContext,
     ws: Option<&Workspace<'_>>,
-    _opts: ReportRebuildsOptions,
+    opts: ReportRebuildsOptions,
 ) -> CargoResult<()> {
-    let Some((log, run_id)) = list_log_files(gctx, ws)?.next() else {
+    let Some((log, run_id)) = find_log_file(gctx, ws, opts.id.as_ref())? else {
         let context = if let Some(ws) = ws {
             format!(" for workspace at `{}`", ws.root().display())
         } else {
             String::new()
         };
-        let title = format!("no sessions found{context}");
-        let note = "run command with `-Z build-analysis` to generate log files";
+        let (title, note) = if let Some(id) = &opts.id {
+            (
+                format!("session `{id}` not found{context}"),
+                "run `cargo report sessions` to list available sessions",
+            )
+        } else {
+            (
+                format!("no sessions found{context}"),
+                "run command with `-Z build-analysis` to generate log files",
+            )
+        };
         let report = [Level::ERROR
             .primary_title(title)
             .element(Level::NOTE.message(note))];
diff --git a/src/cargo/ops/cargo_report/timings.rs b/src/cargo/ops/cargo_report/timings.rs
index b2734716da9..c596f8fa74b 100644
--- a/src/cargo/ops/cargo_report/timings.rs
+++ b/src/cargo/ops/cargo_report/timings.rs
@@ -26,7 +26,7 @@ use crate::core::compiler::timings::report::aggregate_sections;
 use crate::core::compiler::timings::report::compute_concurrency;
 use crate::core::compiler::timings::report::round_to_centisecond;
 use crate::core::compiler::timings::report::write_html;
-use crate::ops::cargo_report::util::list_log_files;
+use crate::ops::cargo_report::util::find_log_file;
 use crate::ops::cargo_report::util::unit_target_description;
 use crate::util::log_message::FingerprintStatus;
 use crate::util::log_message::LogMessage;
@@ -38,6 +38,7 @@ pub struct ReportTimingsOptions<'gctx> {
     /// Whether to attempt to open the browser after the report is generated
     pub open_result: bool,
     pub gctx: &'gctx GlobalContext,
+    pub id: Option<RunId>,
 }
 
 /// Collects sections data for later post-processing through [`aggregate_sections`].
@@ -53,14 +54,23 @@ pub fn report_timings(
     ws: Option<&Workspace<'_>>,
     opts: ReportTimingsOptions<'_>,
 ) -> CargoResult<()> {
-    let Some((log, run_id)) = list_log_files(gctx, ws)?.next() else {
+    let Some((log, run_id)) = find_log_file(gctx, ws, opts.id.as_ref())? else {
         let context = if let Some(ws) = ws {
             format!(" for workspace at `{}`", ws.root().display())
         } else {
             String::new()
         };
-        let title = format!("no sessions found{context}");
-        let note = "run command with `-Z build-analysis` to generate log files";
+        let (title, note) = if let Some(id) = &opts.id {
+            (
+                format!("session `{id}` not found{context}"),
+                "run `cargo report sessions` to list available sessions",
+            )
+        } else {
+            (
+                format!("no sessions found{context}"),
+                "run command with `-Z build-analysis` to generate log files",
+            )
+        };
         let report = [Level::ERROR
             .primary_title(title)
             .element(Level::NOTE.message(note))];
diff --git a/src/cargo/ops/cargo_report/util.rs b/src/cargo/ops/cargo_report/util.rs
index 682eac2aa30..64bf872da0d 100644
--- a/src/cargo/ops/cargo_report/util.rs
+++ b/src/cargo/ops/cargo_report/util.rs
@@ -60,6 +60,24 @@ pub fn list_log_files(
     Ok(Box::new(walk))
 }
 
+pub fn find_log_file(
+    gctx: &GlobalContext,
+    ws: Option<&Workspace<'_>>,
+    id: Option<&RunId>,
+) -> CargoResult<Option<(std::path::PathBuf, RunId)>> {
+    match id {
+        Some(requested_id) => {
+            for (path, run_id) in list_log_files(gctx, ws)? {
+                if run_id.to_string() == requested_id.to_string() {
+                    return Ok(Some((path, run_id)));
+                }
+            }
+            Ok(None)
+        }
+        None => Ok(list_log_files(gctx, ws)?.next()),
+    }
+}
+
 pub fn unit_target_description(target: &Target, mode: CompileMode) -> String {
     // This is pretty similar to how the current `core::compiler::timings`
     // renders `core::manifest::Target`. However, our target is
diff --git a/tests/testsuite/cargo_report_rebuilds/mod.rs b/tests/testsuite/cargo_report_rebuilds/mod.rs
index 2c2c2e0c866..35fda5df3da 100644
--- a/tests/testsuite/cargo_report_rebuilds/mod.rs
+++ b/tests/testsuite/cargo_report_rebuilds/mod.rs
@@ -565,7 +565,7 @@ Root rebuilds:
 }
 
 #[cargo_test]
-fn uses_latest_session_by_default() {
+fn with_session_id() {
     let p = project()
         .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
         .file("src/lib.rs", "")
@@ -577,7 +577,8 @@ fn uses_latest_session_by_default() {
         .masquerade_as_nightly_cargo(&["build-analysis"])
         .run();
 
-    let _ = paths::log_file(0);
+    let first_log = paths::log_file(0);
+    let first_session_id = first_log.file_stem().unwrap().to_str().unwrap();
 
     p.change_file("src/lib.rs", "// touched");
 
@@ -589,20 +590,56 @@ fn uses_latest_session_by_default() {
 
     let _ = paths::log_file(1);
 
-    // Without --id, should use the most recent (second) session which has a rebuild
-    p.cargo("report rebuilds -Zbuild-analysis")
+    // With --id, should use the first session (not the most recent second)
+    p.cargo(&format!(
+        "report rebuilds --id {first_session_id} -Zbuild-analysis"
+    ))
+    .masquerade_as_nightly_cargo(&["build-analysis"])
+    .with_stderr_data(str![[r#"
+Session: [..]
+Status: 0 units rebuilt, 0 cached, 1 new
+
+
+"#]])
+    .run();
+}
+
+#[cargo_test]
+fn session_id_not_found() {
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -Zbuild-analysis")
+        .env("CARGO_BUILD_ANALYSIS_ENABLED", "true")
         .masquerade_as_nightly_cargo(&["build-analysis"])
+        .run();
+
+    p.cargo("report rebuilds --id 20260101T000000000Z-0000000000000000 -Zbuild-analysis")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .with_status(101)
         .with_stderr_data(str![[r#"
-Session: [..]
-Status: 1 unit rebuilt, 0 cached, 0 new
+[ERROR] session `20260101T000000000Z-0000000000000000` not found for workspace at `[ROOT]/foo`
+  |
+  = [NOTE] run `cargo report sessions` to list available sessions
 
-Rebuild impact:
-  root rebuilds: 1 unit
-  cascading:     0 units
+"#]])
+        .run();
+}
 
-Root rebuilds:
-  0. foo@0.0.0 (check): file modified: src/lib.rs
-     impact: no cascading rebuilds
+#[cargo_test]
+fn invalid_session_id_format() {
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("report rebuilds --id invalid-session-id -Zbuild-analysis")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] expect run ID in format `20060724T012128000Z-<16-char-hex>`, got `invalid-session-id`
 
 "#]])
         .run();
diff --git a/tests/testsuite/cargo_report_timings/mod.rs b/tests/testsuite/cargo_report_timings/mod.rs
index 03cd6941318..3f885e19f2a 100644
--- a/tests/testsuite/cargo_report_timings/mod.rs
+++ b/tests/testsuite/cargo_report_timings/mod.rs
@@ -347,7 +347,7 @@ fn with_multiple_targets() {
 }
 
 #[cargo_test]
-fn uses_latest_session_by_default() {
+fn with_session_id() {
     let p = project()
         .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
         .file("src/lib.rs", "")
@@ -370,25 +370,68 @@ fn uses_latest_session_by_default() {
     let second_log = paths::log_file(1);
     let second_session_id = second_log.file_stem().unwrap().to_str().unwrap();
 
-    // Without --id, should use the most recent (second) session
-    p.cargo("report timings -Zbuild-analysis")
-        .masquerade_as_nightly_cargo(&["build-analysis"])
-        .with_stderr_data(str![[r#"
+    // With --id, should use the first session (not the most recent second)
+    p.cargo(&format!(
+        "report timings --id {first_session_id} -Zbuild-analysis"
+    ))
+    .masquerade_as_nightly_cargo(&["build-analysis"])
+    .with_stderr_data(str![[r#"
       Timing report saved to [ROOT]/foo/target/cargo-timings/cargo-timing-[..]T[..]Z-[..].html
 
 "#]])
-        .run();
+    .run();
 
     let timing_files: Vec<_> = p.glob("**/cargo-timing-*.html").collect();
     assert_eq!(timing_files.len(), 1);
     let timing_file = timing_files[0].as_ref().unwrap();
     let filename = timing_file.file_name().unwrap().to_str().unwrap();
     assert!(
-        filename.contains(second_session_id),
-        "Expected timing file to contain second session ID {second_session_id}, got {filename}"
+        filename.contains(first_session_id),
+        "Expected timing file to contain first session ID {first_session_id}, got {filename}"
     );
     assert!(
-        !filename.contains(first_session_id),
-        "Should not contain first session ID {first_session_id}, got {filename}"
+        !filename.contains(second_session_id),
+        "Should not contain second session ID {second_session_id}, got {filename}"
     );
 }
+
+#[cargo_test]
+fn session_id_not_found() {
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -Zbuild-analysis")
+        .env("CARGO_BUILD_ANALYSIS_ENABLED", "true")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .run();
+
+    p.cargo("report timings --id 20260101T000000000Z-0000000000000000 -Zbuild-analysis")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] session `20260101T000000000Z-0000000000000000` not found for workspace at `[ROOT]/foo`
+  |
+  = [NOTE] run `cargo report sessions` to list available sessions
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn invalid_session_id_format() {
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "0.0.0"))
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("report timings --id invalid-session-id -Zbuild-analysis")
+        .masquerade_as_nightly_cargo(&["build-analysis"])
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] expect run ID in format `20060724T012128000Z-<16-char-hex>`, got `invalid-session-id`
+
+"#]])
+        .run();
+}