Make elapsed-since-last an explicit param extractor, not a Duration-kind special case (ticket 038)

Previously a Value-role param with kind: Duration silently meant "elapsed
time since the previous same-labeled event" instead of parsed text — the
same MetricPrimitiveKind::Duration variant meant two unrelated things
depending on which list (label_params vs value_param) referenced it.

Add ParamDeclaration::elapsed_since_last, a third extraction source
alongside capture_group/json_field_path: when set, the param is computed
(via a new resolve_param_value, used uniformly for label_params and
value_param) rather than text-extracted, and declare_param validates it's
paired with kind: Duration. apply_metric_to_log no longer branches on
(role, kind) — it dispatches purely on what each resolved param declares.

Migrate existing profiles: migrate_duration_value_params converts any
Duration-kind param referenced as a metric's value_param into the new
extractor, and resolve_or_create_legacy_param (ticket 037's pre-existing
migration) now carries the same meaning forward for pre-037 inline
Value-role Duration matchers.

Params.tsx offers "Elapsed since previous event" as its own Extraction
option (hides the now-irrelevant message-match-regex/capture-group
fields and forces kind to duration); Metrics.tsx labels such params
accordingly wherever they're listed as label/value param choices.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/api.rs

@@ -244,6 +244,7 @@ async fn declare_param(
                 capture_group: request.capture_group,
                 json_field_path: request.json_field_path,
                 kind: request.kind,
+                elapsed_since_last: request.elapsed_since_last,
             };
             (StatusCode::CREATED, Json(dto)).into_response()
         }

src/data_value.rs

@@ -22,6 +22,17 @@ pub struct ParamDeclaration {
     #[serde(default)]
     pub json_field_path: Option<String>,
     pub kind: MetricPrimitiveKind,
+    /// A third extraction source, alongside `capture_group`/`json_field_path`: when set, this
+    /// param's value is never parsed from text at all — it's computed as elapsed time since the
+    /// previous event that produced the same label set (ticket 038). Takes precedence over
+    /// `json_field_path`/`capture_group` when set, and `message_match_regex`/`capture_group`/
+    /// `json_field_path` are simply unused in that case (`declare_param` doesn't validate them for
+    /// this shape). Only meaningful paired with `kind == Duration`, since elapsed time is the only
+    /// primitive this computation can produce — `declare_param` enforces that pairing.
+    /// `#[serde(default)]` so params declared before this field existed keep parsing as plain
+    /// regex/JSON-field extraction.
+    #[serde(default)]
+    pub elapsed_since_last: bool,
 }
 
 /// Reads a value out of a JSON object by dot-separated field path (e.g. `"foo.bar"`), for
@@ -56,7 +67,11 @@ pub enum MetricPrimitiveKind {
     String,
     Number,
     Timestamp,
-    /// On a metric's Value param this is derived as elapsed time since the previous event (not parsed from text); otherwise it's a literal i64 millisecond count.
+    /// A literal i64 millisecond count. Note this is unrelated to
+    /// `ParamDeclaration::elapsed_since_last`, which is a separate, independent extraction source —
+    /// a param using that source always parses/produces `Duration`, but not every `Duration`-kind
+    /// param is computed that way (one can also just parse a literal duration out of captured text,
+    /// like any other kind).
     Duration,
     /// A standalone param declared with this kind doesn't require (or use) a capture group: its value is
     /// simply whether `message_match_regex` matches the line, case-insensitively or per the regex's own

src/dto.rs

@@ -23,6 +23,8 @@ pub struct ParamDeclarationDto {
     /// See `crate::data_value::ParamDeclaration::json_field_path`.
     pub json_field_path: Option<String>,
     pub kind: MetricPrimitiveKind,
+    /// See `crate::data_value::ParamDeclaration::elapsed_since_last`.
+    pub elapsed_since_last: bool,
 }
 
 #[derive(Debug, Clone, Serialize)]

src/engine.rs

@@ -219,11 +219,19 @@ fn declare_param(
     index: &mut BTreeMap<IndexKey, DataValue>,
     request: DeclareParamRequest,
 ) -> Result<(), String> {
+    // An `elapsed_since_last` param is never text-extracted at all, so `message_match_regex` is
+    // irrelevant to it — but the field always compiles as a regex regardless of extractor (an empty
+    // pattern is valid and matches trivially), so it's harmless to still require it to parse.
     let re = Regex::new(&request.message_match_regex)
         .map_err(|err| format!("invalid regex: {err}"))?;
+    if request.elapsed_since_last && request.kind != MetricPrimitiveKind::Duration {
+        return Err("elapsed_since_last params must declare kind 'duration'".to_string());
+    }
     // A Boolean-kind param never uses a capture group — its value is just whether the regex matched.
+    // Neither does an `elapsed_since_last` param — its value is computed, not captured.
     let uses_json_field = request.json_field_path.as_deref().is_some_and(|path| !path.is_empty());
     if request.kind != MetricPrimitiveKind::Boolean
+        && !request.elapsed_since_last
         && !uses_json_field
         && !re.capture_names().flatten().any(|name| name == request.capture_group)
     {
@@ -371,7 +379,9 @@ fn parse_metric_primitive(raw: &str, kind: MetricPrimitiveKind) -> Option<Metric
                 .ok()
                 .map(MetricPrimitive::Timestamp)
         }
-        // A Value-role param never reaches this branch (see apply_metric_to_log); reached only for a Label-role param or standalone ParamDeclaration, parsed as a literal millisecond count.
+        // An `elapsed_since_last` param never reaches this branch (see `resolve_param_value`, which
+        // computes its value directly rather than extracting/parsing text); reached only for a
+        // Duration-kind param that parses a literal captured/field millisecond count.
         MetricPrimitiveKind::Duration => raw.parse::<i64>().ok().map(MetricPrimitive::Duration),
         // A standalone Boolean param never reaches this branch (see derive_log_params, which derives its
         // value from whether the regex matched rather than parsing captured text); reached only if a metric
@@ -389,11 +399,15 @@ struct ResolvedParam {
     capture_group: String,
     json_field_path: Option<String>,
     kind: MetricPrimitiveKind,
+    /// See `crate::data_value::ParamDeclaration::elapsed_since_last`. When set, `regex`/
+    /// `capture_group`/`json_field_path` above are unused — resolution goes through
+    /// `resolve_param_value`'s computed path instead of `extract_param_raw`.
+    elapsed_since_last: bool,
 }
 
 fn resolved_param_declarations(index: &BTreeMap<IndexKey, DataValue>) -> HashMap<String, ResolvedParam> {
     param_declarations_range(index)
-        .filter_map(|(name, message_match_regex, capture_group, json_field_path, kind)| {
+        .filter_map(|(name, message_match_regex, capture_group, json_field_path, kind, elapsed_since_last)| {
             Regex::new(message_match_regex).ok().map(|regex| {
                 (
                     name.to_string(),
@@ -402,6 +416,7 @@ fn resolved_param_declarations(index: &BTreeMap<IndexKey, DataValue>) -> HashMap
                         capture_group: capture_group.to_string(),
                         json_field_path: json_field_path.map(|path| path.to_string()),
                         kind,
+                        elapsed_since_last,
                     },
                 )
             })
@@ -426,14 +441,50 @@ fn extract_param_raw(
     }
 }
 
+/// Resolves one param's value against `text`/`timestamp`, dispatching purely on what the param
+/// itself declares (`ResolvedParam::elapsed_since_last`) rather than on whether it's being resolved
+/// as a label or as the value — the same resolution is used for both in `apply_metric_to_log`, so
+/// an `elapsed_since_last` param behaves identically regardless of which list references it. An
+/// elapsed-since-last param's value is elapsed time since the previous event that produced
+/// `labels_so_far` under `metric_name`, tracked via a `LastMetricEvent` index entry; its first
+/// occurrence returns `None` (nothing to diff against yet) rather than a bogus zero. Every other
+/// param falls through to ordinary text extraction (regex capture group or JSON field path) parsed
+/// as its declared kind.
+fn resolve_param_value(
+    index: &mut BTreeMap<IndexKey, DataValue>,
+    metric_name: &str,
+    resolved: &ResolvedParam,
+    labels_so_far: &BTreeMap<String, MetricPrimitive>,
+    text: &str,
+    timestamp: Timestamp,
+) -> Option<MetricPrimitive> {
+    if resolved.elapsed_since_last {
+        // Tracked separately from Bucket: the first occurrence for a given label set pushes no
+        // Bucket entry, so the last-event timestamp can't live in Bucket.values.
+        let last_event_key = IndexKey::LastMetricEvent {
+            metric_name: metric_name.to_string(),
+            labels: labels_so_far.clone(),
+        };
+        let previous_timestamp = match index.get(&last_event_key) {
+            Some(DataValue::LastEventTimestamp { timestamp: previous }) => Some(*previous),
+            _ => None,
+        };
+        index.insert(last_event_key, DataValue::LastEventTimestamp { timestamp });
+        return previous_timestamp
+            .map(|previous| MetricPrimitive::Duration(timestamp.as_millis() - previous.as_millis()));
+    }
+
+    let raw = extract_param_raw(text, &resolved.regex, &resolved.capture_group, resolved.json_field_path.as_deref())?;
+    parse_metric_primitive(&raw, resolved.kind)
+}
+
 /// Resolves `label_params`/`value_param` by name against `resolved_params` — a dangling reference
 /// (a param removed after this metric was declared) skips the whole match, same as any other
 /// extraction failure. At most one param is the value param (its primitive becomes the event's
 /// value, otherwise a fixed `1` marker is used); any param failing to match/parse skips the whole
-/// match rather than storing a partial one. A `Duration`-kind value param is elapsed time since the
-/// previous same-labeled event, so the first occurrence pushes no bucket (nothing to diff against)
-/// rather than a bogus zero. When `regex` (the metric's own message-match regex) doesn't match and
-/// `unmatched_labels` is `Some`, an occurrence is still registered with those fixed labels.
+/// match rather than storing a partial one. When `regex` (the metric's own message-match regex)
+/// doesn't match and `unmatched_labels` is `Some`, an occurrence is still registered with those
+/// fixed labels.
 #[allow(clippy::too_many_arguments)]
 fn apply_metric_to_log(
     index: &mut BTreeMap<IndexKey, DataValue>,
@@ -462,65 +513,27 @@ fn apply_metric_to_log(
     let mut labels = BTreeMap::new();
     for param_name in label_params {
         let Some(resolved) = resolved_params.get(param_name) else { return };
-        let Some(raw) = extract_param_raw(
-            text,
-            &resolved.regex,
-            &resolved.capture_group,
-            resolved.json_field_path.as_deref(),
-        ) else {
-            return;
-        };
-        let Some(parsed) = parse_metric_primitive(&raw, resolved.kind) else {
+        let Some(parsed) = resolve_param_value(index, metric_name, resolved, &labels, text, timestamp) else {
             return;
         };
         labels.insert(param_name.clone(), parsed);
     }
 
-    let mut value = None;
-    let mut is_duration = false;
-    if let Some(value_param) = value_param {
-        let Some(resolved) = resolved_params.get(value_param) else { return };
-        if resolved.kind == MetricPrimitiveKind::Duration {
-            is_duration = true;
-        } else {
-            let Some(raw) = extract_param_raw(
-                text,
-                &resolved.regex,
-                &resolved.capture_group,
-                resolved.json_field_path.as_deref(),
-            ) else {
-                return;
-            };
-            let Some(parsed) = parse_metric_primitive(&raw, resolved.kind) else {
-                return;
-            };
-            value = Some(parsed);
-        }
-    }
-
     if let Some(matched_labels) = matched_labels {
         for (name, value) in matched_labels {
             labels.insert(name.clone(), MetricPrimitive::String(value.clone()));
         }
     }
 
-    let value = if is_duration {
-        // Tracked separately from Bucket: a Duration's first occurrence pushes no Bucket entry, so the last-event timestamp can't live in Bucket.values.
-        let last_event_key =
-            IndexKey::LastMetricEvent { metric_name: metric_name.to_string(), labels: labels.clone() };
-        let previous_timestamp = match index.get(&last_event_key) {
-            Some(DataValue::LastEventTimestamp { timestamp: previous }) => Some(*previous),
-            _ => None,
-        };
-        index.insert(last_event_key, DataValue::LastEventTimestamp { timestamp });
-        match previous_timestamp {
-            Some(previous) => {
-                MetricPrimitive::Duration(timestamp.as_millis() - previous.as_millis())
-            }
+    let value = match value_param {
+        Some(value_param) => {
+            let Some(resolved) = resolved_params.get(value_param) else { return };
+            match resolve_param_value(index, metric_name, resolved, &labels, text, timestamp) {
+                Some(parsed) => parsed,
                 None => return,
             }
-    } else {
-        value.unwrap_or(MetricPrimitive::Number(Number::Int(1)))
+        }
+        None => MetricPrimitive::Number(Number::Int(1)),
     };
 
     push_bucket_value(index, metric_name, labels, sequence, timestamp, value);
@@ -943,7 +956,7 @@ fn metric_declarations_range(
 
 fn param_declarations_range(
     index: &BTreeMap<IndexKey, DataValue>,
-) -> impl Iterator<Item = (&str, &str, &str, Option<&str>, MetricPrimitiveKind)> {
+) -> impl Iterator<Item = (&str, &str, &str, Option<&str>, MetricPrimitiveKind, bool)> {
     index
         .range(IndexKey::LogParamSplit..IndexKey::ParamMetricSplit)
         .filter_map(|(key, value)| match (key, value) {
@@ -954,6 +967,7 @@ fn param_declarations_range(
                     capture_group,
                     json_field_path,
                     kind,
+                    elapsed_since_last,
                     ..
                 }),
             ) => Some((
@@ -962,17 +976,23 @@ fn param_declarations_range(
                 capture_group.as_str(),
                 json_field_path.as_deref(),
                 *kind,
+                *elapsed_since_last,
             )),
             _ => None,
         })
 }
 
-/// A param whose stored regex fails to compile (shouldn't happen — `declare_param` validates it) is skipped rather than panicking.
+/// A param whose stored regex fails to compile (shouldn't happen — `declare_param` validates it) is
+/// skipped rather than panicking. An `elapsed_since_last` param is skipped outright — it has no
+/// meaningful reading as a standalone logs-table column (its value depends on which metric's label
+/// set it's grouped under, which only exists once it's referenced by a metric's `label_params`/
+/// `value_param`).
 fn compiled_param_declarations(
     index: &BTreeMap<IndexKey, DataValue>,
 ) -> Vec<(String, Regex, String, Option<String>, MetricPrimitiveKind)> {
     param_declarations_range(index)
-        .filter_map(|(name, message_match_regex, capture_group, json_field_path, kind)| {
+        .filter(|(.., elapsed_since_last)| !elapsed_since_last)
+        .filter_map(|(name, message_match_regex, capture_group, json_field_path, kind, _)| {
             Regex::new(message_match_regex).ok().map(|re| {
                 (
                     name.to_string(),
@@ -1178,12 +1198,15 @@ fn handle_query(
         }
         EngineQuery::ParamDeclarations => {
             let declarations = param_declarations_range(index)
-                .map(|(name, message_match_regex, capture_group, json_field_path, kind)| ParamDeclarationDto {
+                .map(|(name, message_match_regex, capture_group, json_field_path, kind, elapsed_since_last)| {
+                    ParamDeclarationDto {
                         name: name.to_string(),
                         message_match_regex: message_match_regex.to_string(),
                         capture_group: capture_group.to_string(),
                         json_field_path: json_field_path.map(|path| path.to_string()),
                         kind,
+                        elapsed_since_last,
+                    }
                 })
                 .collect();
             EngineResponse::ParamDeclarations(declarations)
@@ -1554,6 +1577,7 @@ mod tests {
             capture_group: capture_group.to_string(),
             json_field_path: None,
             kind: MetricPrimitiveKind::String,
+            elapsed_since_last: false,
         }
     }
 
@@ -1589,6 +1613,34 @@ mod tests {
         assert!(index.is_empty());
     }
 
+    #[test]
+    fn declares_valid_elapsed_since_last_param_without_capture_group() {
+        let mut index = BTreeMap::new();
+        declare_elapsed_since_last_param(&mut index, "gap");
+
+        let declarations: Vec<_> = param_declarations_range(&index).collect();
+        assert_eq!(declarations.len(), 1);
+        assert!(declarations[0].5, "expected elapsed_since_last to be true");
+    }
+
+    #[test]
+    fn rejects_elapsed_since_last_param_with_non_duration_kind() {
+        let mut index = BTreeMap::new();
+        let result = declare_param(
+            &mut index,
+            DeclareParamRequest {
+                name: "gap".to_string(),
+                message_match_regex: ".".to_string(),
+                capture_group: String::new(),
+                json_field_path: None,
+                kind: MetricPrimitiveKind::String,
+                elapsed_since_last: true,
+            },
+        );
+        assert!(result.is_err());
+        assert!(index.is_empty());
+    }
+
     #[test]
     fn declares_valid_param_from_json_field_path() {
         let mut index = BTreeMap::new();
@@ -1602,6 +1654,7 @@ mod tests {
                 capture_group: String::new(),
                 json_field_path: Some("foo.bar".to_string()),
                 kind: MetricPrimitiveKind::String,
+                elapsed_since_last: false,
             },
         );
         assert!(result.is_ok());
@@ -1622,6 +1675,7 @@ mod tests {
                 capture_group: String::new(),
                 json_field_path: Some("foo.bar".to_string()),
                 kind: MetricPrimitiveKind::Number,
+                elapsed_since_last: false,
             },
         )
         .unwrap();
@@ -1737,6 +1791,26 @@ mod tests {
                 capture_group: capture_group.to_string(),
                 json_field_path: None,
                 kind,
+                elapsed_since_last: false,
+            },
+        )
+        .unwrap();
+    }
+
+    /// Declares a standalone `elapsed_since_last` param (ticket 038) — computed as elapsed time
+    /// since the previous event sharing whatever label set it's grouped under, never text-extracted.
+    /// `message_match_regex`/`capture_group` are unused for this extractor but still need to satisfy
+    /// `declare_param`'s baseline "is this a valid regex" check, so an always-matching `"."` is fine.
+    fn declare_elapsed_since_last_param(index: &mut BTreeMap<IndexKey, DataValue>, name: &str) {
+        declare_param(
+            index,
+            DeclareParamRequest {
+                name: name.to_string(),
+                message_match_regex: ".".to_string(),
+                capture_group: String::new(),
+                json_field_path: None,
+                kind: MetricPrimitiveKind::Duration,
+                elapsed_since_last: true,
             },
         )
         .unwrap();
@@ -1957,6 +2031,7 @@ mod tests {
                 capture_group: String::new(),
                 json_field_path: Some("request.path".to_string()),
                 kind: MetricPrimitiveKind::String,
+                elapsed_since_last: false,
             },
         )
         .unwrap();
@@ -2579,10 +2654,7 @@ mod tests {
     fn declare_login_duration_metric(index: &mut BTreeMap<IndexKey, DataValue>) {
         let pattern = r"user=(?P<user>\S+) logged in";
         declare_test_param(index, "user", pattern, "user", MetricPrimitiveKind::String);
-        // A Duration-kind value param never actually extracts via its own capture group (see
-        // `apply_metric_to_log`) — its regex/capture-group only need to satisfy `declare_param`'s
-        // validation, so reusing "user" here is harmless.
-        declare_test_param(index, "gap", pattern, "user", MetricPrimitiveKind::Duration);
+        declare_elapsed_since_last_param(index, "gap");
         declare_metric(index, metric_request_with_value("login_gap", pattern, vec!["user"], Some("gap"))).unwrap();
     }
 

src/profile.rs

@@ -69,6 +69,11 @@ pub struct ParamDeclarationRecord {
     #[serde(default)]
     pub json_field_path: Option<String>,
     pub kind: ProfileMetricPrimitiveKind,
+    /// See `crate::data_value::ParamDeclaration::elapsed_since_last`; `#[serde(default)]` so
+    /// profiles saved before ticket 038 (where a `Duration`-kind value param implicitly meant this)
+    /// still load — `migrate_duration_value_params` below converts those.
+    #[serde(default)]
+    pub elapsed_since_last: bool,
 }
 
 impl From<&DeclareParamRequest> for ParamDeclarationRecord {
@@ -79,6 +84,7 @@ impl From<&DeclareParamRequest> for ParamDeclarationRecord {
             capture_group: request.capture_group.clone(),
             json_field_path: request.json_field_path.clone(),
             kind: request.kind.into(),
+            elapsed_since_last: request.elapsed_since_last,
         }
     }
 }
@@ -91,6 +97,7 @@ impl From<&ParamDeclarationRecord> for DeclareParamRequest {
             capture_group: record.capture_group.clone(),
             json_field_path: record.json_field_path.clone(),
             kind: record.kind.into(),
+            elapsed_since_last: record.elapsed_since_last,
         }
     }
 }
@@ -236,11 +243,44 @@ fn resolve_or_create_legacy_param(
             capture_group: matcher.capture_group.clone(),
             json_field_path: matcher.json_field_path.clone(),
             kind: matcher.kind,
+            // Pre-037, a Value-role Duration matcher implicitly meant "elapsed time since the
+            // previous same-labeled event" (ticket 038's now-explicit `elapsed_since_last`) — carry
+            // that meaning forward rather than dropping it into a meaningless literal-parse matcher.
+            elapsed_since_last: matcher.role == LegacyParamRole::Value
+                && matcher.kind == ProfileMetricPrimitiveKind::Duration,
         });
     }
     implicit_name
 }
 
+/// Migrates every already-declared `Duration`-kind param that's referenced as some metric's
+/// `value_param` into the explicit `elapsed_since_last` extractor (ticket 038). Before ticket 038, a
+/// `Duration`-kind value param implicitly meant "elapsed time since the previous same-labeled
+/// event" (see `crate::engine::apply_metric_to_log`'s pre-038 history) — this migration makes that
+/// meaning explicit so `Duration` kind no longer has a role-dependent special case. Only params
+/// actually referenced as *some* metric's value param are converted: a `Duration`-kind param used
+/// purely as a label param already meant a literal parsed millisecond count, and keeps meaning that.
+/// Runs after `migrate_legacy_metric_params` (which populates `value_param` from pre-037
+/// `legacy_params` and already sets `elapsed_since_last` correctly for those via
+/// `resolve_or_create_legacy_param`), so this only has new work to do for profiles saved between
+/// ticket 037 and ticket 038 landing, where `value_param` already pointed at a bare `Duration`-kind
+/// param straight from the unified shape. Returns whether anything changed.
+fn migrate_duration_value_params(profile: &mut Profile) -> bool {
+    let value_param_names: std::collections::HashSet<&str> =
+        profile.metrics.iter().filter_map(|metric| metric.value_param.as_deref()).collect();
+    let mut migrated = false;
+    for param in &mut profile.params {
+        if !param.elapsed_since_last
+            && param.kind == ProfileMetricPrimitiveKind::Duration
+            && value_param_names.contains(param.name.as_str())
+        {
+            param.elapsed_since_last = true;
+            migrated = true;
+        }
+    }
+    migrated
+}
+
 /// A chart type a dashboard panel can render; kept in sync with the frontend's `'bar' | 'line' | 'pie'` union (see `webapp/src/lib/api.ts`).
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "lowercase")]
@@ -485,9 +525,17 @@ impl ProfileStore {
         // load; persisting the result isn't required for correctness (migration re-runs
         // identically next time), but it's done here anyway so the on-disk file only carries the
         // legacy `"params"` shape once, on the very next save.
+        //
+        // Ticket 038: follows on immediately after — once `migrate_legacy_metric_params` has
+        // resolved every metric's `value_param`, convert any `Duration`-kind param still referenced
+        // that way into the explicit `elapsed_since_last` extractor, so a profile saved either before
+        // 037 (inline matchers) or between 037 and 038 (bare `value_param` reference) ends up
+        // behaving identically under the new, explicit shape.
         let mut any_migrated = false;
         for profile in &mut store.profiles {
-            if migrate_legacy_metric_params(profile) {
+            let legacy_migrated = migrate_legacy_metric_params(profile);
+            let duration_migrated = migrate_duration_value_params(profile);
+            if legacy_migrated || duration_migrated {
                 any_migrated = true;
             }
         }
@@ -766,6 +814,7 @@ mod tests {
             capture_group: capture_group.to_string(),
             json_field_path: None,
             kind: MetricPrimitiveKind::String,
+            elapsed_since_last: false,
         }
     }
 
@@ -1283,6 +1332,84 @@ mod tests {
         });
     }
 
+    /// Acceptance criterion (ticket 038): a pre-037 legacy profile whose inline `Value`-role matcher
+    /// used `kind: "duration"` (the old "elapsed time since previous event" special case) migrates
+    /// into a standalone param with the new explicit `elapsed_since_last: true` extractor, not just a
+    /// bare `Duration`-kind param that would now (post-038) mean a literal parsed millisecond count.
+    #[test]
+    fn migrates_legacy_value_role_duration_matcher_into_elapsed_since_last_param() {
+        let dir = tempfile::tempdir().unwrap();
+        let store_path = dir.path().join("profiles.json");
+        write_legacy_profiles_json(
+            &store_path,
+            r#"[
+                {
+                    "name": "legacy",
+                    "params": [],
+                    "metrics": [
+                        {
+                            "name": "login_gap",
+                            "message_match_regex": "user=(?P<user>\\S+) logged in",
+                            "params": [
+                                { "param_name": "user", "capture_group": "user", "kind": "string", "role": "label" },
+                                { "param_name": "gap", "capture_group": "user", "kind": "duration", "role": "value" }
+                            ]
+                        }
+                    ],
+                    "dashboards": []
+                }
+            ]"#,
+        );
+
+        let store = ProfileStore::load(&store_path);
+        let profile = store.get("legacy").unwrap();
+
+        let gap_param = profile.params.iter().find(|p| p.name == "login_gap.gap").unwrap();
+        assert!(matches!(gap_param.kind, ProfileMetricPrimitiveKind::Duration));
+        assert!(gap_param.elapsed_since_last);
+        assert_eq!(profile.metrics[0].value_param, Some("login_gap.gap".to_string()));
+    }
+
+    /// Acceptance criterion (ticket 038): a profile saved after ticket 037 landed but before ticket
+    /// 038 — so it already carries the unified `label_params`/`value_param` shape rather than legacy
+    /// inline matchers, with a bare `Duration`-kind param referenced as a metric's `value_param` — is
+    /// migrated the same way: the param gains `elapsed_since_last: true` so its behavior is unchanged
+    /// (elapsed time since the previous same-labeled event) under the new explicit extractor.
+    #[test]
+    fn migrates_post_037_bare_duration_value_param_into_elapsed_since_last() {
+        let dir = tempfile::tempdir().unwrap();
+        let store_path = dir.path().join("profiles.json");
+        write_legacy_profiles_json(
+            &store_path,
+            r#"[
+                {
+                    "name": "legacy",
+                    "params": [
+                        { "name": "user", "message_match_regex": "user=(?P<user>\\S+) logged in", "capture_group": "user", "kind": "string" },
+                        { "name": "gap", "message_match_regex": "user=(?P<user>\\S+) logged in", "capture_group": "user", "kind": "duration" }
+                    ],
+                    "metrics": [
+                        {
+                            "name": "login_gap",
+                            "message_match_regex": "user=(?P<user>\\S+) logged in",
+                            "label_params": ["user"],
+                            "value_param": "gap"
+                        }
+                    ],
+                    "dashboards": []
+                }
+            ]"#,
+        );
+
+        let store = ProfileStore::load(&store_path);
+        let profile = store.get("legacy").unwrap();
+
+        let gap_param = profile.params.iter().find(|p| p.name == "gap").unwrap();
+        assert!(gap_param.elapsed_since_last);
+        // A Duration-kind param not referenced as any value_param is left untouched (still a literal parse).
+        assert!(!profile.params.iter().find(|p| p.name == "user").unwrap().elapsed_since_last);
+    }
+
     /// When a bare-named param already declared on the profile matches a legacy matcher's shape
     /// exactly (same regex/capture-group/kind), migration reuses it by name instead of creating a
     /// redundant `<metric>.<param>` declaration.

webapp/src/components/Metrics.tsx

@@ -254,7 +254,7 @@ export function Metrics({ range, activePreset }: { range: TimeRange | null; acti
                       onChange={() => toggleLabelParam(p.name)}
                     />
                     <span className="font-mono">{p.name}</span>
-                    <span className="text-text-faint">({p.kind})</span>
+                    <span className="text-text-faint">({p.elapsed_since_last ? 'elapsed since previous' : p.kind})</span>
                   </label>
                 ))
               )}
@@ -263,7 +263,7 @@ export function Metrics({ range, activePreset }: { range: TimeRange | null; acti
           <div>
             <label
               className="mb-1 block text-xs text-text-subtle"
-              title="At most one param becomes the counter's per-event captured value (used by histogram()/gauge()) instead of a label. A 'duration'-kind value param is derived automatically — elapsed time since the previous match with the same labels — rather than parsed from its capture group."
+              title="At most one param becomes the counter's per-event captured value (used by histogram()/gauge()) instead of a label. An 'elapsed since previous event' param (declared in the Params panel) is computed automatically — elapsed time since the previous match with the same labels — rather than parsed from its capture group."
             >
               Value param
             </label>
@@ -275,7 +275,7 @@ export function Metrics({ range, activePreset }: { range: TimeRange | null; acti
               <option value="">(none — occurrence count)</option>
               {params.map((p) => (
                 <option key={p.name} value={p.name}>
-                  {p.name} ({p.kind})
+                  {p.name} ({p.elapsed_since_last ? 'elapsed since previous' : p.kind})
                 </option>
               ))}
             </select>

webapp/src/components/Params.tsx

@@ -13,7 +13,11 @@ import { RegexHelpIcon } from './RegexHelpIcon'
 
 const EMPTY_KIND: ParamDeclaration['kind'] = 'string'
 
-type ExtractionSource = 'regex' | 'json_field'
+// 'elapsed_since_last' is always offered, regardless of what the message match regex looks like —
+// unlike 'json_field' (which only makes sense once the sample line looks like a JSON object), this
+// param's value is never extracted from text at all: it's elapsed time since the previous event
+// sharing the same label set (ticket 038).
+type ExtractionSource = 'regex' | 'json_field' | 'elapsed_since_last'
 
 /// A JSON *object* (not an array/scalar) parses cleanly and pretty-prints with braces — the same
 /// shape a JSON field path can be read from, so this is the "line looks like JSON" gate that
@@ -41,23 +45,39 @@ export function Params({ params, onDeclared }: { params: ParamDeclaration[]; onD
   const [collapsed, setCollapsed] = useSectionCollapsed('params')
 
   const isBoolean = kind === 'boolean'
+  const isElapsedSinceLast = source === 'elapsed_since_last'
   // The message match regex doubles as a sample line here so a pasted JSON log line unlocks the
-  // "JSON field" extraction option — regex-based params are unaffected either way.
+  // "JSON field" extraction option — regex-based params are unaffected either way. Not consulted at
+  // all for "elapsed since previous event", which is always offered.
   const detectedJson = useMemo(() => looksLikeJsonObject(regex), [regex])
-  const effectiveSource: ExtractionSource = detectedJson && !isBoolean ? source : 'regex'
+  const effectiveSource: ExtractionSource = isElapsedSinceLast
+    ? 'elapsed_since_last'
+    : detectedJson && !isBoolean
+      ? source
+      : 'regex'
+
+  function handleSourceChange(next: ExtractionSource) {
+    setSource(next)
+    // "Elapsed since previous event" always produces a Duration value — never text-extracted — so
+    // there's no separate kind to pick; force it rather than leaving a stale/mismatched kind selected.
+    if (next === 'elapsed_since_last') setKind('duration')
+  }
 
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault()
     setSubmitError(null)
     setSubmitting(true)
     try {
-      // A boolean-kind param never uses a capture group — its value is just whether the regex matched.
+      // A boolean-kind param never uses a capture group — its value is just whether the regex
+      // matched. An elapsed-since-last param never uses a capture group, JSON field path, or even
+      // its own message match regex — its value is computed, not extracted from text at all.
       await declareParam({
         name,
-        message_match_regex: regex,
-        capture_group: isBoolean || effectiveSource === 'json_field' ? '' : captureGroup,
+        message_match_regex: effectiveSource === 'elapsed_since_last' ? '' : regex,
+        capture_group: isBoolean || effectiveSource !== 'regex' ? '' : captureGroup,
         json_field_path: effectiveSource === 'json_field' ? jsonFieldPath : null,
-        kind,
+        kind: effectiveSource === 'elapsed_since_last' ? 'duration' : kind,
+        elapsed_since_last: effectiveSource === 'elapsed_since_last',
       })
       setName('')
       setRegex('')
@@ -104,6 +124,7 @@ export function Params({ params, onDeclared }: { params: ParamDeclaration[]; onD
               required
             />
           </div>
+          {!isElapsedSinceLast && (
             <div>
               <label className="mb-1 flex items-center gap-1 text-xs text-text-subtle">
                 Message match regex
@@ -117,20 +138,25 @@ export function Params({ params, onDeclared }: { params: ParamDeclaration[]; onD
                 required
               />
             </div>
-          {detectedJson && !isBoolean && (
+          )}
           <div>
             <label className="mb-1 block text-xs text-text-subtle">Extraction</label>
             <select
               className="w-full rounded border border-border-soft bg-surface-1 px-2 py-1 text-xs text-text focus:border-border-strong focus:outline-none"
               value={source}
-                onChange={(e) => setSource(e.target.value as ExtractionSource)}
+              onChange={(e) => handleSourceChange(e.target.value as ExtractionSource)}
             >
               <option value="regex">Regex capture group</option>
-                <option value="json_field">JSON field</option>
+              {detectedJson && !isBoolean && <option value="json_field">JSON field</option>}
+              <option value="elapsed_since_last">Elapsed since previous event</option>
             </select>
           </div>
-          )}
-          {effectiveSource === 'json_field' ? (
+          {isElapsedSinceLast ? (
+            <p className="text-xs text-text-faint">
+              Computed automatically — elapsed time since the previous event that shares the same
+              labels wherever this param is used, not parsed from the log line.
+            </p>
+          ) : effectiveSource === 'json_field' ? (
             <div className="flex items-end gap-1">
               <div className="min-w-0 flex-1">
                 <label className="mb-1 block text-xs text-text-subtle">JSON field path</label>
@@ -199,10 +225,12 @@ export function Params({ params, onDeclared }: { params: ParamDeclaration[]; onD
               <span
                 key={p.name}
                 className="rounded border border-border-soft px-2 py-1 text-xs text-text-secondary"
-                title={p.message_match_regex}
+                title={p.elapsed_since_last ? undefined : p.message_match_regex}
               >
                 {p.name} ({p.kind}
-                {p.kind === 'boolean'
+                {p.elapsed_since_last
+                  ? ', elapsed since previous'
+                  : p.kind === 'boolean'
                     ? ''
                     : `, ${p.json_field_path ? `json field ${p.json_field_path}` : `group ${p.capture_group}`}`}
                 )

webapp/src/lib/api.ts

@@ -30,6 +30,8 @@ export interface ParamDeclaration {
   /** Alternative to `capture_group`: a dot-separated JSON field path (e.g. "foo.bar"), read from lines that parse as a JSON object. When set (non-empty), takes precedence over `capture_group`. */
   json_field_path?: string | null
   kind: 'string' | 'number' | 'timestamp' | 'duration' | 'boolean'
+  /** A third extraction source, distinct from `capture_group`/`json_field_path`: when true, this param's value is never parsed from text — it's computed as elapsed time since the previous event sharing the same label set. Only meaningful paired with `kind: 'duration'`. */
+  elapsed_since_last?: boolean
 }
 
 export interface TimelineBucket {