diff --git a/CHANGELOG.md b/CHANGELOG.md
index 882e75b..27f1594 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   explanation": it prints the current state, the requested state, the valid next state(s) from the
   state machine, and the shortest legal path to the requested state when reachable (e.g. `draft -> done`
   is rejected with the hint `draft -> approved -> in-progress -> done`). No file is mutated on refusal.
+- `/sd:bug` Phase 3 no longer assumes a confirmed root cause always arrives. The investigation loop
+  previously said "Continue until one is CONFIRMED" with no exit, so a bug whose every hypothesis is
+  rejected/inconclusive had no defined stopping point. Added Gate 3a (hypothesis tree exhausted): the
+  loop now terminates on a CONFIRMED hypothesis OR an exhausted tree, and the exhausted case STOPs and
+  asks the user to re-enumerate (with new evidence), add observability, or abort as "root cause not
+  found" - never guessing a fix from an unconfirmed tree.
 - `install/install.sh` marked executable (mode `100755`, matching `uninstall.sh`); it was `100644`,
   so the documented `./install/install.sh` invocation failed with "Permission denied" on a fresh
   Linux checkout. Surfaced by the new CI round-trip.
diff --git a/commands/bug.md b/commands/bug.md
index d07647e..953d1c3 100644
--- a/commands/bug.md
+++ b/commands/bug.md
@@ -108,7 +108,29 @@ If the user insists on proceeding without repro, log a constitution exception to
    - Result: CONFIRMED / REJECTED / INCONCLUSIVE.
    - Append result with evidence pointers (file:line, log lines, query results) to `03-decisions.md`.
    - Document REJECTED hypotheses with FULL reasoning - this is knowledge preservation for future similar bugs.
-   - Continue until one is CONFIRMED.
+   - Terminate the loop when EITHER one hypothesis is CONFIRMED (proceed to Gate 3) OR every enumerated
+     hypothesis is exhausted - all REJECTED, or only INCONCLUSIVE ones remain with no new evidence to act
+     on (go to Gate 3a). Never guess a fix from an unconfirmed tree.
+
+### ⛔ Gate 3a - Hypothesis tree exhausted (no confirmed root cause)
+
+Reached ONLY when the loop ends with no CONFIRMED hypothesis. STOP. Do NOT proceed to a fix - a fix on an
+unconfirmed root cause risks treating a symptom.
+
+Append the exhausted tree (every hypothesis with its REJECTED / INCONCLUSIVE verdict and reasoning) to
+`.specs/BUG-<arg>/03-decisions.md` - this is the knowledge record for the next investigation.
+
+Ask:
+
+> All <N> hypotheses were rejected or inconclusive; no root cause confirmed. How do you want to proceed?
+> (re-enumerate / observe / abort)
+
+- `re-enumerate` -> return to Phase 3 step 1 with the new evidence/telemetry that justifies fresh
+  hypotheses. Do not re-run identical hypotheses.
+- `observe` -> add observability (logging, tracing, metrics), reproduce again to gather evidence, then
+  re-enumerate. Log the gap to `05-retro.md`.
+- `abort` -> set status and close the spec as "root cause not found"; record the exhausted tree as the
+  outcome in `05-retro.md`.
 
 ### ⛔ Gate 3 - Root cause confirmed