fix(mcp): scope detect_changes seed detection to changed line ranges

detect_collect_seeds treated every definition in a changed file as a
BFS seed, regardless of which lines actually changed. A one-line edit
inside a single method seeded every other definition in the file too,
producing an impact report an order of magnitude larger than what the
edit actually touched.

Fetch `git diff --unified=0` hunks (cbm_parse_hunks already existed in
pass_gitdiff.c but had no caller) alongside the existing file list, and
scope seeds to definitions whose line range overlaps a hunk. Any
failure fetching hunks (new/untracked files, a transient git error)
falls back to the previous whole-file behavior, so this is a precision
improvement with no new failure mode.

Verified on microsoft/qlib (an unrelated third-party repo): a same-
line-count edit inside one method now seeds 2 symbols (the method +
its containing class) instead of 25 (every definition in the 24-def
file), matching the equivalent output of a different code-graph tool
(GitNexus) on the identical scenario. Reproduced on a second file with
a different definition count (42 defs -> seeds 43 before the fix, 2
after) to rule out coincidence.

cbm_changed_hunk_t / cbm_parse_hunks move from pipeline_internal.h
(explicitly not a public header) to pipeline.h, since detect_changes
now needs them from src/mcp/mcp.c. cbm_detect_node_in_hunks is exposed
non-static via mcp_internal.h, matching this file's existing white-box
test-hook pattern, so the overlap logic has a direct unit test
independent of the git/subprocess/index plumbing around it.

Fixes #1363

Signed-off-by: lishixiang <lishixiang@gmail.com>

src/mcp/mcp.c

@@ -9736,19 +9736,54 @@ static int mcp_run_shell_command_cancellable(cbm_mcp_server_t *srv, const char *
     return contained ? 0 : -1;
 }
 
-/* Collect BFS seed ids: every symbol DEFINED in a changed file (everything but
- * the structural container labels — those have no CALLS edges). These anchor
- * the multi-source impact traversal. */
+/* Does `node`'s line range overlap any recorded hunk for `file`? Used to scope
+ * seed detection to the actually-changed lines rather than the whole file.
+ * Non-static (declared in mcp_internal.h) so tests can exercise the overlap
+ * logic directly, matching this file's existing white-box test hooks. */
+bool cbm_detect_node_in_hunks(const cbm_node_t *node, const cbm_changed_hunk_t *hunks,
+                              int hunk_count, const char *file) {
+    for (int h = 0; h < hunk_count; h++) {
+        if (strcmp(hunks[h].path, file) == 0 && node->start_line <= hunks[h].end_line &&
+            node->end_line >= hunks[h].start_line) {
+            return true;
+        }
+    }
+    return false;
+}
+
+/* Collect BFS seed ids for one changed file (everything but the structural
+ * container labels — those have no CALLS edges). These anchor the
+ * multi-source impact traversal.
+ *
+ * When `hunks` has at least one entry for `file`, only definitions whose line
+ * range overlaps a hunk become seeds — a one-line edit inside a single
+ * function no longer seeds every other definition in the file. When no hunk
+ * is recorded for `file` (a brand-new/untracked file has no comparable
+ * "before" state, or the hunk fetch failed/was skipped), every non-container
+ * definition in the file is a seed — the previous, whole-file behavior — so
+ * this is a precision improvement, not a new failure mode. */
 static void detect_collect_seeds(cbm_store_t *store, const char *project, const char *file,
-                                 int64_t **seeds, int *n, int *cap) {
+                                 const cbm_changed_hunk_t *hunks, int hunk_count, int64_t **seeds,
+                                 int *n, int *cap) {
     cbm_node_t *nodes = NULL;
     int ncount = 0;
     cbm_store_find_nodes_by_file(store, project, file, &nodes, &ncount);
+    bool has_hunks_for_file = false;
+    for (int h = 0; h < hunk_count; h++) {
+        if (strcmp(hunks[h].path, file) == 0) {
+            has_hunks_for_file = true;
+            break;
+        }
+    }
     for (int i = 0; i < ncount; i++) {
         const char *lb = nodes[i].label;
         if (lb && strcmp(lb, "File") != 0 && strcmp(lb, "Folder") != 0 &&
             strcmp(lb, "Project") != 0 && strcmp(lb, "Module") != 0 && strcmp(lb, "Package") != 0 &&
             strcmp(lb, "Section") != 0) {
+            if (has_hunks_for_file &&
+                !cbm_detect_node_in_hunks(&nodes[i], hunks, hunk_count, file)) {
+                continue;
+            }
             if (*n >= *cap) {
                 *cap = *cap ? *cap * 2 : 16;
                 *seeds = safe_realloc(*seeds, (size_t)*cap * sizeof(int64_t));
@@ -10035,6 +10070,55 @@ static char *handle_detect_changes(cbm_mcp_server_t *srv, const char *args) {
     int seed_count = 0;
     int seed_cap = 0;
 
+    /* Hunk line ranges (unified=0 diff), used to scope seed detection to the
+     * actually-changed lines instead of every definition in a changed file
+     * (see detect_collect_seeds). Best-effort: any failure here just leaves
+     * `hunks` empty and every file falls back to its previous whole-file
+     * seeding — this is a precision improvement, not a correctness
+     * dependency, so it is never treated as a request-level failure. */
+    cbm_changed_hunk_t *hunks = NULL;
+    int hunk_count = 0;
+    if (want_symbols) {
+        char hunk_cmd[CBM_SZ_2K];
+#ifdef _WIN32
+        snprintf(hunk_cmd, sizeof(hunk_cmd),
+                 "git -C \"%s\" diff --unified=0 \"%s\"...HEAD 2>NUL & "
+                 "git -C \"%s\" diff --unified=0 2>NUL",
+                 root_path, base_branch, root_path);
+#else
+        snprintf(hunk_cmd, sizeof(hunk_cmd),
+                 "{ git -C '%s' diff --unified=0 '%s'...HEAD 2>/dev/null; "
+                 "git -C '%s' diff --unified=0 2>/dev/null; }",
+                 root_path, base_branch, root_path);
+#endif
+        char hunk_output_path[CBM_SZ_2K] = {0};
+        cbm_proc_result_t hunk_result = {0};
+        int hunk_run =
+            mcp_run_shell_command_cancellable(srv, hunk_cmd, hunk_output_path, &hunk_result);
+        bool hunk_cancelled = hunk_result.cancellation_requested || mcp_request_cancelled(srv);
+        FILE *hfp = (!hunk_cancelled && hunk_run == 0) ? cbm_fopen(hunk_output_path, "rb") : NULL;
+        if (hfp) {
+            (void)fseek(hfp, 0, SEEK_END);
+            long hsz = ftell(hfp);
+            if (hsz > 0) {
+                (void)fseek(hfp, 0, SEEK_SET);
+                char *hbuf = malloc((size_t)hsz + SKIP_ONE);
+                if (hbuf) {
+                    size_t hread = fread(hbuf, SKIP_ONE, (size_t)hsz, hfp);
+                    hbuf[hread] = '\0';
+                    enum { HUNK_CAP = 4096 };
+                    hunks = safe_realloc(NULL, (size_t)HUNK_CAP * sizeof(cbm_changed_hunk_t));
+                    hunk_count = cbm_parse_hunks(hbuf, hunks, HUNK_CAP);
+                    free(hbuf);
+                }
+            }
+            (void)fclose(hfp);
+        }
+        if (hunk_output_path[0]) {
+            (void)cbm_unlink(hunk_output_path);
+        }
+    }
+
     char line[CBM_SZ_1K];
     while (fgets(line, sizeof(line), fp)) {
         size_t len = strlen(line);
@@ -10077,7 +10161,8 @@ static char *handle_detect_changes(cbm_mcp_server_t *srv, const char *args) {
         }
         files[file_count++] = heap_strdup(path_line);
         if (want_symbols) {
-            detect_collect_seeds(store, project, path_line, &seeds, &seed_count, &seed_cap);
+            detect_collect_seeds(store, project, path_line, hunks, hunk_count, &seeds, &seed_count,
+                                 &seed_cap);
         }
     }
     (void)fclose(fp);
@@ -10123,6 +10208,7 @@ static char *handle_detect_changes(cbm_mcp_server_t *srv, const char *args) {
             }
             free(files);
             free(seeds);
+            free(hunks);
             free(direction);
             free(root_path);
             free(project);
@@ -10280,6 +10366,7 @@ static char *handle_detect_changes(cbm_mcp_server_t *srv, const char *args) {
     }
     free(files);
     free(seeds);
+    free(hunks);
     free(direction);
     free(root_path);
     free(project);

src/mcp/mcp_internal.h

@@ -2,6 +2,8 @@
 #define CBM_MCP_INTERNAL_H
 
 #include "mcp/mcp.h"
+#include "pipeline/pipeline.h" /* cbm_changed_hunk_t */
+#include "store/store.h"       /* cbm_node_t */
 
 /* White-box fault injection for deterministic cross-platform quarantine
  * safety tests. This header is internal and is not part of the MCP API. */
@@ -31,4 +33,10 @@ enum { CBM_MCP_DEFAULT_AUTO_INDEX_LIMIT = 50000 };
 bool cbm_mcp_auto_index_within_file_limit(const char *root_path, int file_limit,
                                           int *file_count_out);
 
+/* detect_changes seed scoping (#1363): does `node`'s line range overlap any
+ * recorded hunk for `file`? Exposed for direct unit testing of the overlap
+ * logic, independent of the git/subprocess/index plumbing around it. */
+bool cbm_detect_node_in_hunks(const cbm_node_t *node, const cbm_changed_hunk_t *hunks,
+                              int hunk_count, const char *file);
+
 #endif

src/pipeline/pipeline.h

@@ -19,7 +19,8 @@
 #include <stdint.h>
 #include <stdatomic.h>
 
-#include "discover/discover.h" /* cbm_ignored_file_t (#963) */
+#include "discover/discover.h"    /* cbm_ignored_file_t (#963) */
+#include "foundation/constants.h" /* CBM_SZ_512 */
 
 /* Forward declarations */
 typedef struct cbm_store cbm_store_t;
@@ -290,4 +291,20 @@ cbm_fuzzy_result_t cbm_registry_fuzzy_resolve(const cbm_registry_t *r, const cha
 
 const char *cbm_confidence_band(double score);
 
+/* ── Git diff hunks (pass_gitdiff.c) ──────────────────────────────
+ * Public (unlike the rest of pipeline_internal.h) because detect_changes
+ * (src/mcp/mcp.c) scopes seed detection to changed line ranges, not just
+ * changed files. */
+
+typedef struct {
+    char path[CBM_SZ_512];
+    int start_line;
+    int end_line;
+} cbm_changed_hunk_t;
+
+/* Parse `git diff --unified=0` output into per-hunk (path, start_line,
+ * end_line) entries — end_line is the last new-side line the hunk touches.
+ * Returns count written to out (capped at max_out). */
+int cbm_parse_hunks(const char *output, cbm_changed_hunk_t *out, int max_out);
+
 #endif /* CBM_PIPELINE_H */

src/pipeline/pipeline_internal.h

@@ -281,18 +281,13 @@ typedef struct {
     char old_path[CBM_SZ_512]; /* non-empty only for renames */
 } cbm_changed_file_t;
 
-typedef struct {
-    char path[CBM_SZ_512];
-    int start_line;
-    int end_line;
-} cbm_changed_hunk_t;
+/* cbm_changed_hunk_t + cbm_parse_hunks moved to pipeline.h (public — consumed
+ * by src/mcp/mcp.c's detect_changes for line-scoped seed detection). Visible
+ * here via the `#include "pipeline/pipeline.h"` above. */
 
 /* Parse git diff --name-status output. Returns count written to out. */
 int cbm_parse_name_status(const char *output, cbm_changed_file_t *out, int max_out);
 
-/* Parse git diff --unified=0 output. Returns count written to out. */
-int cbm_parse_hunks(const char *output, cbm_changed_hunk_t *out, int max_out);
-
 /* Parse "start,count" or "start" → (start, count). */
 void cbm_parse_range(const char *s, int *out_start, int *out_count);
 

tests/test_mcp.c

@@ -6136,6 +6136,109 @@ TEST(tool_detect_changes_contained_commands_clean_up_error_and_success) {
     PASS();
 }
 
+/* Regression test for issue #1363: detect_changes seeded every definition in
+ * a changed file instead of just the ones whose line range overlaps the diff
+ * hunk. cbm_detect_node_in_hunks is the overlap primitive; this exercises it
+ * directly, independent of the git/subprocess/index plumbing around it. */
+TEST(detect_changes_node_in_hunks_overlap_issue1363) {
+    cbm_changed_hunk_t hunks[2] = {
+        {.path = "pkg/mod.py", .start_line = 10, .end_line = 12},
+        {.path = "pkg/other.py", .start_line = 1, .end_line = 1},
+    };
+
+    cbm_node_t inside = {.start_line = 8, .end_line = 15};
+    ASSERT(cbm_detect_node_in_hunks(&inside, hunks, 2, "pkg/mod.py"));
+
+    cbm_node_t exact = {.start_line = 10, .end_line = 12};
+    ASSERT(cbm_detect_node_in_hunks(&exact, hunks, 2, "pkg/mod.py"));
+
+    cbm_node_t touches_edge = {.start_line = 12, .end_line = 20};
+    ASSERT(cbm_detect_node_in_hunks(&touches_edge, hunks, 2, "pkg/mod.py"));
+
+    cbm_node_t before = {.start_line = 1, .end_line = 9};
+    ASSERT(!cbm_detect_node_in_hunks(&before, hunks, 2, "pkg/mod.py"));
+
+    cbm_node_t after = {.start_line = 13, .end_line = 20};
+    ASSERT(!cbm_detect_node_in_hunks(&after, hunks, 2, "pkg/mod.py"));
+
+    /* Same line range, different file — must not match. */
+    cbm_node_t wrong_file = {.start_line = 10, .end_line = 12};
+    ASSERT(!cbm_detect_node_in_hunks(&wrong_file, hunks, 2, "pkg/unrelated.py"));
+
+    PASS();
+}
+
+/* End-to-end regression test for issue #1363: a same-line-count edit inside
+ * one function must seed only that function, not every definition in the
+ * file. A flat file with two independent top-level functions (no enclosing
+ * class) makes this unambiguous — before the fix, editing foo() also seeded
+ * bar() because seeding was scoped to the whole changed file. */
+TEST(detect_changes_seeds_only_touched_symbol_issue1363) {
+    char repo[512];
+    snprintf(repo, sizeof(repo), "%s/cbm-detect-seed-scope-XXXXXX", cbm_tmpdir());
+    if (!cbm_mkdtemp(repo)) {
+        FAIL("cbm_mkdtemp failed");
+    }
+
+    char src[600];
+    snprintf(src, sizeof(src), "%s/mod.py", repo);
+    ASSERT_EQ(th_write_file(src, "def foo():\n"
+                                 "    x = 1\n"
+                                 "    return x\n"
+                                 "\n"
+                                 "\n"
+                                 "def bar():\n"
+                                 "    y = 2\n"
+                                 "    return y\n"),
+             0);
+
+    char cmd[1200];
+    snprintf(cmd, sizeof(cmd),
+             "cd '%s' && git init -q && git config user.email t@t.com && "
+             "git config user.name t && git add -A && git commit -q -m init",
+             repo);
+    if (system(cmd) != 0) {
+        th_rmtree(repo);
+        FAIL("git fixture setup failed");
+    }
+
+    cbm_mcp_server_t *srv = cbm_mcp_server_new(NULL);
+    char idx_args[700];
+    snprintf(idx_args, sizeof(idx_args), "{\"repo_path\":\"%s\",\"mode\":\"full\"}", repo);
+    char *idx_resp = cbm_mcp_handle_tool(srv, "index_repository", idx_args);
+    ASSERT_NOT_NULL(idx_resp);
+    free(idx_resp);
+
+    /* Same-line-count in-place edit inside foo() only; bar() is untouched. */
+    ASSERT_EQ(th_write_file(src, "def foo():\n"
+                                 "    x = 11\n"
+                                 "    return x\n"
+                                 "\n"
+                                 "\n"
+                                 "def bar():\n"
+                                 "    y = 2\n"
+                                 "    return y\n"),
+             0);
+
+    char *project = cbm_project_name_from_path(repo);
+    ASSERT_NOT_NULL(project);
+    char dc_args[700];
+    snprintf(dc_args, sizeof(dc_args), "{\"project\":\"%s\",\"depth\":1}", project);
+    char *dc_resp = cbm_mcp_handle_tool(srv, "detect_changes", dc_args);
+    ASSERT_NOT_NULL(dc_resp);
+    /* cbm_mcp_handle_tool wraps the tree text in a JSON string, so a literal
+     * newline in the source becomes the two-character `\n` escape sequence
+     * in dc_resp's actual bytes — match that, not a real newline. */
+    ASSERT_NOT_NULL(strstr(dc_resp, "seed_symbols: 1\\n"));
+    ASSERT_NULL(strstr(dc_resp, "bar"));
+
+    free(dc_resp);
+    free(project);
+    cbm_mcp_server_free(srv);
+    th_rmtree(repo);
+    PASS();
+}
+
 TEST(tool_ingest_traces_basic) {
     cbm_mcp_server_t *srv = cbm_mcp_server_new(NULL);
 
@@ -9854,6 +9957,8 @@ SUITE(mcp) {
     RUN_TEST(tool_manage_adr_get_accepts_symlink_path);
     RUN_TEST(tool_detect_changes_not_found_rich_error);
     RUN_TEST(tool_detect_changes_contained_commands_clean_up_error_and_success);
+    RUN_TEST(detect_changes_node_in_hunks_overlap_issue1363);
+    RUN_TEST(detect_changes_seeds_only_touched_symbol_issue1363);
     RUN_TEST(tool_ingest_traces_basic);
     RUN_TEST(tool_ingest_traces_empty);