feat(git): warn about co-change partners missing from diff and PR

crates/tracedecay-contracts/src/retrieval/git_context_surface.rs

@@ -217,6 +217,28 @@ pub struct GitContextSymbolV1 {
     pub line: u32,
 }
 
+/// A file that historically changes with `partner_of` but is missing from the
+/// change set under review, e.g. a migration beside its schema.
+#[derive(Clone, Debug, Deserialize, JsonSchema, PartialEq, Eq, Serialize)]
+#[serde(deny_unknown_fields)]
+pub struct CoChangePartnerV1 {
+    pub file: String,
+    pub partner_of: String,
+    /// Commits that changed both `partner_of` and `file`.
+    pub co_changes: u64,
+    /// Commits that changed `partner_of`.
+    pub partner_of_changes: u64,
+}
+
+/// Why co-change mining produced no answer, so an empty
+/// `missing_co_change_partners` does not read as "no partner is missing".
+#[derive(Clone, Debug, Deserialize, JsonSchema, PartialEq, Eq, Serialize)]
+#[serde(deny_unknown_fields)]
+pub struct CoChangeUnavailableV1 {
+    pub reason: String,
+    pub retryable: bool,
+}
+
 #[derive(Clone, Debug, Deserialize, JsonSchema, PartialEq, Eq, Serialize)]
 #[serde(deny_unknown_fields)]
 pub struct DiffContextResultV1 {
@@ -228,6 +250,12 @@ pub struct DiffContextResultV1 {
     /// still unexplored.
     pub impact_complete: bool,
     pub affected_tests: Vec<String>,
+    /// Files that usually change with a changed file, per bounded Git
+    /// history, but are absent from this change set.
+    pub missing_co_change_partners: Vec<CoChangePartnerV1>,
+    /// Present when Git history could not be mined for co-change partners.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub co_change_unavailable: Option<CoChangeUnavailableV1>,
     /// The worktree verdict a served graph read opens with.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub freshness: Option<PrimitiveSearchFreshnessV1>,
@@ -490,6 +518,12 @@ pub struct PrContextCompleteV1 {
     pub commits: Vec<GitCommitSubjectV1>,
     pub files_changed: usize,
     pub changes: Vec<GitFileChangeV1>,
+    /// Files that usually change with a changed file, per bounded Git
+    /// history, but are absent from this change set.
+    pub missing_co_change_partners: Vec<CoChangePartnerV1>,
+    /// Present when Git history could not be mined for co-change partners.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub co_change_unavailable: Option<CoChangeUnavailableV1>,
     pub symbols_added: usize,
     pub symbols_removed: usize,
     pub symbols_modified: usize,
@@ -526,6 +560,12 @@ pub struct PrContextSymbolsUnavailableV1 {
     pub commits: Vec<GitCommitSubjectV1>,
     pub files_changed: usize,
     pub changes: Vec<GitFileChangeV1>,
+    /// Files that usually change with a changed file, per bounded Git
+    /// history, but are absent from this change set.
+    pub missing_co_change_partners: Vec<CoChangePartnerV1>,
+    /// Present when Git history could not be mined for co-change partners.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub co_change_unavailable: Option<CoChangeUnavailableV1>,
     pub symbols_added: usize,
     pub symbols_removed: usize,
     pub symbols_modified: usize,
@@ -555,6 +595,12 @@ pub struct PrContextGraphPendingV1 {
     pub commits: Vec<GitCommitSubjectV1>,
     pub files_changed: usize,
     pub changes: Vec<GitFileChangeV1>,
+    /// Files that usually change with a changed file, per bounded Git
+    /// history, but are absent from this change set.
+    pub missing_co_change_partners: Vec<CoChangePartnerV1>,
+    /// Present when Git history could not be mined for co-change partners.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub co_change_unavailable: Option<CoChangeUnavailableV1>,
     pub symbols_added: usize,
     pub symbols_modified: usize,
     pub added: Vec<PrSymbolEntryV1>,

crates/tracedecay-contracts/src/retrieval/mod.rs

@@ -162,16 +162,17 @@ pub use git_context_surface::{
     BranchSearchHitV1, BranchSearchPageV1, BranchSearchResultV1, BranchSearchSurfaceRequestV1,
     BranchSearchUnavailableV1, BranchSnapshotEntryV1, BranchSymbolChangeV1, BranchSymbolV1,
     ChangelogCompleteV1, ChangelogPartialV1, ChangelogResultV1, ChangelogSurfaceRequestV1,
-    CommitCategoryV1, CommitContextResultV1, CommitContextSummaryV1, CommitContextSurfaceRequestV1,
-    CommitFileRoleV1, CommitSymbolEntryV1, CommitSymbolV1, ConfigSummaryKindV1, ConfigSummaryV1,
-    DiffContextResultV1, DiffContextSurfaceRequestV1, GitCommitSubjectV1, GitComparedSymbolV1,
-    GitContextSymbolV1, GitFileChangeStatusV1, GitFileChangeV1, GitFileRoleV1, GitPageStatusV1,
-    GitReadCompleteV1, GitReadPartialV1, GitReadUnavailableV1, GitReferenceLimitV1,
-    GitResultLimitV1, GitToolErrorKindV1, GitToolErrorV1, GitToolFailureV1, GitToolOperationV1,
-    PrAnalysisCoverageV1, PrContextCompleteV1, PrContextGraphPendingV1, PrContextResultV1,
-    PrContextSurfaceRequestV1, PrContextSymbolsUnavailableV1, PrCoverageSelectionV1,
-    PrSelectionCoverageV1, PrSymbolChangesCompleteV1, PrSymbolEntryV1, PrSymbolPageV1,
-    PrSymbolSelectionV1, SymbolChangesCompleteV1, SymbolChangesUnavailableV1,
+    CoChangePartnerV1, CoChangeUnavailableV1, CommitCategoryV1, CommitContextResultV1,
+    CommitContextSummaryV1, CommitContextSurfaceRequestV1, CommitFileRoleV1, CommitSymbolEntryV1,
+    CommitSymbolV1, ConfigSummaryKindV1, ConfigSummaryV1, DiffContextResultV1,
+    DiffContextSurfaceRequestV1, GitCommitSubjectV1, GitComparedSymbolV1, GitContextSymbolV1,
+    GitFileChangeStatusV1, GitFileChangeV1, GitFileRoleV1, GitPageStatusV1, GitReadCompleteV1,
+    GitReadPartialV1, GitReadUnavailableV1, GitReferenceLimitV1, GitResultLimitV1,
+    GitToolErrorKindV1, GitToolErrorV1, GitToolFailureV1, GitToolOperationV1, PrAnalysisCoverageV1,
+    PrContextCompleteV1, PrContextGraphPendingV1, PrContextResultV1, PrContextSurfaceRequestV1,
+    PrContextSymbolsUnavailableV1, PrCoverageSelectionV1, PrSelectionCoverageV1,
+    PrSymbolChangesCompleteV1, PrSymbolEntryV1, PrSymbolPageV1, PrSymbolSelectionV1,
+    SymbolChangesCompleteV1, SymbolChangesUnavailableV1,
 };
 pub use git_topology_anchor::{
     GitTopologyAnchorAuthority, GitTopologyAnchorAuthorityError, GitTopologyAnchorFuture,

crates/tracedecay-mcp/src/handlers/git/context.rs

@@ -16,14 +16,15 @@ use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, Ordering};
 use tracedecay_code_index::graph_projection::CodeGraphSymbolSummaryV1;
 use tracedecay_contracts::retrieval::{
-    ChangelogCompleteV1, ChangelogPartialV1, ChangelogSurfaceRequestV1, CommitCategoryV1,
-    CommitContextSummaryV1, CommitContextSurfaceRequestV1, CommitFileRoleV1, CommitSymbolEntryV1,
-    CommitSymbolV1, ConfigSummaryKindV1, ConfigSummaryV1, DiffContextResultV1,
-    DiffContextSurfaceRequestV1, GitComparedSymbolV1, GitContextSymbolV1, GitReadCompleteV1,
-    GitReadPartialV1, GitReadUnavailableV1, PrAnalysisCoverageV1, PrContextCompleteV1,
-    PrContextGraphPendingV1, PrContextSurfaceRequestV1, PrContextSymbolsUnavailableV1,
-    PrCoverageSelectionV1, PrSelectionCoverageV1, PrSymbolChangesCompleteV1, PrSymbolEntryV1,
-    PrSymbolPageV1, PrSymbolSelectionV1, SymbolChangesCompleteV1, SymbolChangesUnavailableV1,
+    ChangelogCompleteV1, ChangelogPartialV1, ChangelogSurfaceRequestV1, CoChangePartnerV1,
+    CoChangeUnavailableV1, CommitCategoryV1, CommitContextSummaryV1, CommitContextSurfaceRequestV1,
+    CommitFileRoleV1, CommitSymbolEntryV1, CommitSymbolV1, ConfigSummaryKindV1, ConfigSummaryV1,
+    DiffContextResultV1, DiffContextSurfaceRequestV1, GitComparedSymbolV1, GitContextSymbolV1,
+    GitReadCompleteV1, GitReadPartialV1, GitReadUnavailableV1, PrAnalysisCoverageV1,
+    PrContextCompleteV1, PrContextGraphPendingV1, PrContextSurfaceRequestV1,
+    PrContextSymbolsUnavailableV1, PrCoverageSelectionV1, PrSelectionCoverageV1,
+    PrSymbolChangesCompleteV1, PrSymbolEntryV1, PrSymbolPageV1, PrSymbolSelectionV1,
+    SymbolChangesCompleteV1, SymbolChangesUnavailableV1,
 };
 use tracedecay_contracts::{InvocationAnalyticsV1, PrContextAnalyticsV1, PrContextStageTimingsV1};
 use tracedecay_domain::{RelationEdgeKindV1, SymbolOccurrenceId};
@@ -305,6 +306,84 @@ fn context_symbol(symbol: &CodeGraphSymbolSummaryV1) -> Result<GitContextSymbolV
     })
 }
 
+/// Co-change partners missing from a change set. Mining is enrichment: when
+/// it fails the primary context still answers and says why partners are absent.
+struct CoChangeEvidence {
+    missing: Vec<CoChangePartnerV1>,
+    unavailable: Option<CoChangeUnavailableV1>,
+}
+
+struct CancelGitReadOnDrop(tracedecay_runtime_core::cancellation::CancellationToken);
+
+impl Drop for CancelGitReadOnDrop {
+    fn drop(&mut self) {
+        self.0.cancel();
+    }
+}
+
+async fn co_change_evidence(
+    ctx: &McpToolContext<'_>,
+    history: String,
+    tree: String,
+    changed_files: Vec<String>,
+) -> CoChangeEvidence {
+    let cancel = tracedecay_runtime_core::cancellation::CancellationToken::new();
+    let _cancel_on_drop = CancelGitReadOnDrop(cancel.clone());
+    let mut bounds = tracedecay_runtime_core::git::GitCommandBounds {
+        cancel: Some(cancel.clone()),
+        ..Default::default()
+    };
+    if let Some(deadline) = ctx.deadline() {
+        let remaining =
+            tracedecay_daemon_protocol::deadline_remaining(deadline).unwrap_or_default();
+        bounds.deadline = bounds.deadline.min(std::time::Instant::now() + remaining);
+    }
+    let root = ctx.project_root().to_path_buf();
+    let mut worker = tokio::task::spawn_blocking(move || {
+        tracedecay_runtime_core::git::co_change::missing_co_change_partners(
+            &root,
+            &history,
+            &tree,
+            &changed_files,
+            &bounds,
+        )
+    });
+    let joined = match ctx.cancellation() {
+        Some(signal) => tokio::select! {
+            biased;
+            joined = &mut worker => joined,
+            () = signal.cancelled() => {
+                cancel.cancel();
+                worker.await
+            }
+        },
+        None => worker.await,
+    };
+    let unavailable = |reason: String, retryable: bool| CoChangeEvidence {
+        missing: Vec::new(),
+        unavailable: Some(CoChangeUnavailableV1 { reason, retryable }),
+    };
+    match joined {
+        Ok(Ok(partners)) => CoChangeEvidence {
+            missing: partners
+                .into_iter()
+                .map(|partner| CoChangePartnerV1 {
+                    file: partner.file,
+                    partner_of: partner.partner_of,
+                    co_changes: partner.co_changes as u64,
+                    partner_of_changes: partner.partner_of_changes as u64,
+                })
+                .collect(),
+            unavailable: None,
+        },
+        Ok(Err(error)) => {
+            let retryable = !matches!(error, TraceDecayError::HostCliUnavailable { .. });
+            unavailable(error.to_string(), retryable)
+        }
+        Err(join_error) => unavailable(format!("co-change task failed: {join_error}"), true),
+    }
+}
+
 fn all_symbols_in_files(
     graph: &VerifiedGraphQuery,
     files: &HashSet<String>,
@@ -574,6 +653,12 @@ where
     let mut tests_sorted: Vec<String> = affected_tests.into_iter().collect();
     tests_sorted.sort();
 
+    let co_change = tracing::Instrument::instrument(
+        co_change_evidence(ctx, "HEAD".to_owned(), "HEAD".to_owned(), files.clone()),
+        tracing::trace_span!("mcp.git.diff_context.co_change"),
+    )
+    .await;
+
     let touched_files = unique_file_paths(
         all_touched_files
             .iter()
@@ -588,6 +673,8 @@ where
         impacted_symbols,
         impact_complete: impacted.complete,
         affected_tests: tests_sorted,
+        missing_co_change_partners: co_change.missing,
+        co_change_unavailable: co_change.unavailable,
         freshness: None,
     };
     Ok(graph_tool_completion(
@@ -1038,6 +1125,7 @@ struct PrContextGitEvidence {
     merge_base: String,
     commits: Vec<GitCommitSubjectV1>,
     changes: Vec<GitFileChangeV1>,
+    co_change: CoChangeEvidence,
 }
 
 impl PrContextGitEvidence {
@@ -1059,6 +1147,8 @@ impl PrContextGitEvidence {
             commits: self.commits,
             files_changed: self.changes.len(),
             changes: self.changes,
+            missing_co_change_partners: self.co_change.missing,
+            co_change_unavailable: self.co_change.unavailable,
             symbols_added: 0,
             symbols_removed: 0,
             symbols_modified: 0,
@@ -1142,6 +1232,19 @@ where
     });
     let changed_files: Vec<String> = changes.iter().map(|change| change.path.clone()).collect();
     let changed_paths = changed_files.iter().cloned().collect::<HashSet<_>>();
+    // History up to the merge base is the evidence; the compared change set
+    // is what it is checked against, and partners must survive at head.
+    let co_change = tracing::Instrument::instrument(
+        co_change_evidence(
+            ctx,
+            merge_base.clone(),
+            head_oid.clone(),
+            changed_files.clone(),
+        ),
+        tracing::trace_span!("mcp.pr_context.co_change"),
+    )
+    .await;
+    controls.checkpoint()?;
 
     let maximum_symbols = request
         .maximum_symbols
@@ -1157,6 +1260,7 @@ where
         merge_base,
         commits,
         changes,
+        co_change,
     };
 
     let stage_started = std::time::Instant::now();
@@ -1205,6 +1309,8 @@ where
                 commits: evidence.commits,
                 files_changed: evidence.changes.len(),
                 changes: evidence.changes,
+                missing_co_change_partners: evidence.co_change.missing,
+                co_change_unavailable: evidence.co_change.unavailable,
                 symbols_added: 0,
                 symbols_modified: 0,
                 added: Vec::new(),
@@ -1549,6 +1655,8 @@ where
         commits: evidence.commits,
         files_changed: evidence.changes.len(),
         changes: evidence.changes,
+        missing_co_change_partners: evidence.co_change.missing,
+        co_change_unavailable: evidence.co_change.unavailable,
         symbols_added: added.len(),
         symbols_removed: removed.len(),
         symbols_modified: modified.len(),

crates/tracedecay-runtime-core/src/git.rs

@@ -13,6 +13,7 @@
 //! recovery, and reads where exact porcelain semantics remain the authority.
 
 pub mod churn;
+pub mod co_change;
 
 use std::ffi::{OsStr, OsString};
 #[cfg(unix)]