diff --git a/orchestrator/README.md b/orchestrator/README.md
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--- a/orchestrator/README.md
+++ b/orchestrator/README.md
@@ -1 +1,47 @@
-SubmitQueue Orchestrator
+# SubmitQueue Orchestrator
+
+The orchestrator drives a request through a pipeline of queues (topics). Each queue
+is consumed by one step controller that does its work and publishes to the next
+queue. Processing is at-least-once; controllers are idempotent and retry on failure.
+
+## Pipeline state machine
+
+Each state below is a queue (topic); each edge is a publish from one stage to the
+next, labelled with what triggers the transition.
+
+```mermaid
+stateDiagram-v2
+    [*] --> start: land request (from gateway)
+
+    start --> validate: request persisted, URIs claimed
+    validate --> batch: validated (not duplicate, mergeable)
+    batch --> score: grouped into batch
+    score --> speculate: batch scored
+
+    speculate --> build: start speculation
+    build --> buildsignal: build triggered
+    buildsignal --> speculate: build result received
+
+    speculate --> merge: dependencies succeeded
+    merge --> speculate: re-plan dependents
+
+    merge --> conclude: merged (success or conflict)
+    speculate --> conclude: dependency failed / self-heal
+
+    conclude --> [*]: request Landed / Error
+
+    start --> log: audit event (async)
+    score --> log: audit event (async)
+    log --> [*]
+```
+
+Notes:
+
+- **Feedback loops:** `buildsignal` and `merge` both re-publish to `speculate`, which
+  is a state machine that advances a batch one step per delivery (waiting on
+  in-flight dependencies, then merging or failing).
+- **Self-healing:** `speculate` and `merge` re-fan-out to `conclude` when a batch is
+  already terminal, covering any publish that was lost under at-least-once delivery.
+- **`log`** is an async audit sink fed by stages as they progress (currently `start`
+  and `score`); it persists request-log entries and does not feed back into the
+  pipeline.