diff --git a/src/mcp/mcp.c b/src/mcp/mcp.c
index d3aa3bf3..3b61335c 100644
--- a/src/mcp/mcp.c
+++ b/src/mcp/mcp.c
@@ -430,7 +430,8 @@ static const tool_def_t TOOLS[] = {
      "{\"type\":\"object\",\"properties\":{\"project\":{\"type\":\"string\"},\"scope\":{\"type\":"
      "\"string\"},\"depth\":{\"type\":\"integer\",\"default\":2},\"base_branch\":{\"type\":"
      "\"string\",\"default\":\"main\"},\"since\":{\"type\":\"string\",\"description\":"
-     "\"Git ref or date to compare from (e.g. HEAD~5, v0.5.0, 2026-01-01)\"}},\"required\":"
+     "\"Git ref or tag to compare from (e.g. HEAD~5, v0.5.0). Diffs <ref>...HEAD.\"}},"
+     "\"required\":"
      "[\"project\"]}"},
 
     {"manage_adr", "Create or update Architecture Decision Records",
@@ -3865,12 +3866,25 @@ static void detect_add_impacted_symbols(cbm_store_t *store, const char *project,
 static char *handle_detect_changes(cbm_mcp_server_t *srv, const char *args) {
     char *project = cbm_mcp_get_string_arg(args, "project");
     char *base_branch = cbm_mcp_get_string_arg(args, "base_branch");
+    char *since = cbm_mcp_get_string_arg(args, "since");
     char *scope = cbm_mcp_get_string_arg(args, "scope");
     int depth = cbm_mcp_get_int_arg(args, "depth", MCP_DEFAULT_BFS_DEPTH);
 
     /* scope: "files" = just changed files, "symbols" = files + symbols (default) */
     bool want_symbols = !scope || strcmp(scope, "symbols") == 0 || strcmp(scope, "impact") == 0;
 
+    /* `since` (e.g. "HEAD~10", "v0.5.0") is the documented diff base but was
+     * previously parsed and never used: it takes precedence over base_branch.
+     * Route it through base_branch so the shared shell-arg validation and the
+     * existing `<base>...HEAD` (three-dot) diff apply unchanged — `since` thus
+     * adopts the same merge-base semantics base_branch already uses. */
+    if (since && since[0]) {
+        free(base_branch);
+        base_branch = since; /* transfer ownership */
+        since = NULL;
+    }
+    free(since); /* no-op after the swap (since is NULL); frees it otherwise */
+
     if (!base_branch) {
         base_branch = heap_strdup("main");
     }
diff --git a/tests/test_incremental.c b/tests/test_incremental.c
index c210d543..5a082705 100644
--- a/tests/test_incremental.c
+++ b/tests/test_incremental.c
@@ -1763,6 +1763,34 @@ TEST(tool_detect_changes_custom_branch) {
     PASS();
 }
 
+/* Regression: `since` was advertised in the schema but ignored by the handler;
+ * it must be honored as the diff base. Fixture is a --depth=1 shallow clone, so
+ * HEAD~N won't resolve — use HEAD for a valid (empty) diff. */
+TEST(tool_detect_changes_since) {
+    double ms;
+    char *r = call_tool_timed("detect_changes", &ms, "{\"project\":\"%s\",\"since\":\"HEAD\"}",
+                              g_project);
+    TOOL_OK(r, ms);
+    ASSERT(resp_has_key(r, "changed_files"));
+    free(r);
+    PASS();
+}
+
+/* Regression: `since` must take precedence over base_branch. A valid since plus a
+ * bogus base_branch must still succeed (proving since won) and must not reference
+ * the bogus branch. */
+TEST(tool_detect_changes_since_precedence) {
+    double ms;
+    char *r = call_tool_timed(
+        "detect_changes", &ms,
+        "{\"project\":\"%s\",\"since\":\"HEAD\",\"base_branch\":\"no-such-branch-xyz\"}",
+        g_project);
+    TOOL_OK(r, ms);
+    ASSERT(strstr(r, "no-such-branch-xyz") == NULL);
+    free(r);
+    PASS();
+}
+
 TEST(tool_detect_changes_depth) {
     double ms;
     char *r = call_tool_timed("detect_changes", &ms, "{\"project\":\"%s\",\"depth\":5}", g_project);
@@ -2956,6 +2984,8 @@ SUITE(incremental) {
     /* Phase 15: detect_changes */
     RUN_TEST(tool_detect_changes_default);
     RUN_TEST(tool_detect_changes_custom_branch);
+    RUN_TEST(tool_detect_changes_since);
+    RUN_TEST(tool_detect_changes_since_precedence);
     RUN_TEST(tool_detect_changes_depth);
 
     /* Phase 16: manage_adr */