Sample run: A2A unicast¶

Two scripted listeners — copilot and codex — take the same demo-env DIDs as the SLIM collab run, but no SLIM node and no paid coding CLI. The profile bin is a2a-grpc-stdio.py: PING → PONG, and one stub REPLACE per hop. avatar owns the file and starts hop 1 with delegate --to did:key:…. The finishing listener is the A2A client for NEXT.

This page is one live trio from 9 September 2026. The same script was run three times (TRANSPORT=grpc, jsonrpc, http+json). The locator scheme and listen port changed; the DIDs, dest-DID reject, URL move, token path, and fifo cargo test did not. How to start the script is in the A2A unicast demo.


Problem	`fifo` — SOLVED after hop 4 on every binding
Tests	2 / 2 (`empty_new`, `push_pop_order`)
Who wrote	copilot ×2 · codex ×2
Failed delegates	0 (coding hops); dest-DID override rejected as designed
Bindings	gRPC · JSON-RPC · HTTP+JSON

Listeners¶

list --local after register (DIDs truncated; they match demo-env.sh). Copilot later moves to the second port; the DID stays.

# gRPC
copilot  did=did:key:z6MkmJUc…MnwT  grpc://127.0.0.1:50151
codex    did=did:key:z6MkmaFF…uGr6  grpc://127.0.0.1:50152

# JSON-RPC
copilot  did=did:key:z6MkmJUc…MnwT  jsonrpc://127.0.0.1:51151
codex    did=did:key:z6MkmaFF…uGr6  jsonrpc://127.0.0.1:51152

# HTTP+JSON
copilot  did=did:key:z6MkmJUc…MnwT  http+json://127.0.0.1:52151
codex    did=did:key:z6MkmaFF…uGr6  http+json://127.0.0.1:52152

avatar only delegates the coding prompt (and the three locator checks). After NEXT <peer>, that same register process shares the turn. DONE does not send a listener handoff.

Locator checks (before fifo)¶

These three checks ran on every binding. The gRPC transcript is below; JSON-RPC and HTTP+JSON printed the same text with jsonrpc://127.0.0.1:51151 / :51161 or http+json://127.0.0.1:52151 / :52161.

Echo by DID¶

delegate --to did:key:z6MkmJUc… with no --a2a-url. The lease locator is looked up.

Delegating task … to 'copilot' (did=did:key:z6MkmJUc…MnwT) via grpc://127.0.0.1:50151...
Response from 'copilot' (34ms):
PONG

Dest DID mismatch¶

Same URL, a DID that is not the listener. The task must not run.

Delegating task … to 'did:key:zWrongDestinationDid…' via grpc://127.0.0.1:50151...
Response from 'did:key:zWrongDestinationDid…' (3ms):
destination DID did:key:zWrongDestinationDid… is not this agent
(did:key:z6MkmJUc…MnwT); URL is only a locator

No PONG. Two agents can share a locator; a2a-dst-did selects who runs.

Same DID after URL change¶

Copilot is stopped and re-registered on :50161 (:51161 / :52161 on the other bindings). list --local then shows the new URI. delegate --to the same DID, still without --a2a-url:

copilot moved grpc://127.0.0.1:50151 → grpc://127.0.0.1:50161
Delegating task … to 'copilot' (did=did:key:z6MkmJUc…MnwT) via grpc://127.0.0.1:50161...
Response from 'copilot' (29ms):
PONG

fifo — 4 hops¶

The crate is the same stub as run-a2a-grpc-demo.sh: push / pop / len / is_empty are no-ops. Each hop replaces one stub. The scripted binary always names the other peer (or DONE on is_empty).

Hop	Agent	What landed	A2A next
1	copilot	`push` → `self.items.push(item)`	codex
2	codex	`pop` → `drain(0..len.min(1)).next()`	copilot
3	copilot	`len` → `self.items.len()`	codex
4	codex	`is_empty` → `self.items.is_empty()`	`DONE` · SOLVED

Token path (identical on all three bindings):

avatar ──delegate──► copilot ──NEXT──► codex
                                   ──NEXT──► copilot
                                   ──NEXT──► codex ──DONE · SOLVED

Hop 1 — copilot¶

REPLACE 11
    pub fn push(&mut self, item: T) { self.items.push(item); }
NEXT codex

-    pub fn push(&mut self, _item: T) {}
+    pub fn push(&mut self, item: T) { self.items.push(item); }

Hop 2 — codex¶

REPLACE 14
        self.items.drain(0..self.items.len().min(1)).next()
NEXT copilot

     pub fn pop(&mut self) -> Option<T> {
-        None
+        self.items.drain(0..self.items.len().min(1)).next()
     }

Hop 3 — copilot¶

REPLACE 18
        self.items.len()
NEXT codex

     pub fn len(&self) -> usize {
-        0
+        self.items.len()
     }

Hop 4 — codex¶

REPLACE 22
        self.items.is_empty()
DONE

     pub fn is_empty(&self) -> bool {
-        false
+        self.items.is_empty()
     }

cargo test for fifo: 2 passed.

Impl after hop 4 (tests unchanged):

    pub fn push(&mut self, item: T) { self.items.push(item); }

    pub fn pop(&mut self) -> Option<T> {
        self.items.drain(0..self.items.len().min(1)).next()
    }

    pub fn len(&self) -> usize {
        self.items.len()
    }

    pub fn is_empty(&self) -> bool {
        self.items.is_empty()
    }

What these runs showed¶

Register and list --local print a locator URI whose scheme is the binding (grpc://, jsonrpc://, http+json://).
delegate --to did:key:… looks up that locator. --a2a-url is an override only.
A task aimed at another DID on the same URL is rejected; the listener does not run it.
Moving the listen port keeps the DID. The next delegate --to that DID follows the new URI.
The two-line cap still applies (REPLACE + one body line). The scripted hops stay inside it.
DONE with a green cargo test ends the problem on hop 4.

The paid-CLI token path on the same bindings is TRANSPORT=grpc|jsonrpc|http+json PROBLEM=fifo on the round-robin Rust demo. That run was not repeated for this page.

Reproduce¶

cargo build -p agntcy-shadi-cli -p agntcy-agentbridge-cli
bash docs/content/demos/run-a2a-grpc-demo.sh
TRANSPORT=jsonrpc bash docs/content/demos/run-a2a-grpc-demo.sh
TRANSPORT=http+json bash docs/content/demos/run-a2a-grpc-demo.sh

No paid CLI is required. Unset SLIM_ENDPOINT (the script does) so register does not dual-listen. Raw per-turn files from a live run land under /tmp/shadi-a2a-grpc-demo.*/logs (fifo-turns.log, echo.raw, mismatch.raw, moved.raw, *.reply).