diff --git a/workspace/main.py b/workspace/main.py
index 5c09b134..85e891e2 100644
--- a/workspace/main.py
+++ b/workspace/main.py
@@ -437,13 +437,23 @@ async def main():  # pragma: no cover
             )
         async def _send_initial_prompt():
             """Wait for server to be ready, then send initial_prompt as self-message."""
-            # Wait for the A2A server to accept connections
+            # Wait for the A2A server to accept connections.
+            # Use the SDK's own constant for the well-known path so this
+            # probe and the route mounted by create_agent_card_routes()
+            # never drift apart. Pre-fix this hardcoded the pre-1.x
+            # well-known path string; a2a-sdk 1.x renamed it (the
+            # canonical value lives in a2a.utils.constants now), so
+            # the probe got 404 every attempt and fell through to
+            # "server not ready after 30s, skipping" even though the
+            # server was actually serving fine. Net effect: every
+            # workspace silently dropped its `initial_prompt`.
+            from a2a.utils.constants import AGENT_CARD_WELL_KNOWN_PATH
             ready = False
             for attempt in range(30):
                 await asyncio.sleep(1)
                 try:
                     async with httpx.AsyncClient(timeout=5.0) as client:
-                        resp = await client.get(f"http://127.0.0.1:{port}/.well-known/agent.json")
+                        resp = await client.get(f"http://127.0.0.1:{port}{AGENT_CARD_WELL_KNOWN_PATH}")
                         if resp.status_code == 200:
                             ready = True
                             break
diff --git a/workspace/tests/test_agent_card_well_known_path.py b/workspace/tests/test_agent_card_well_known_path.py
new file mode 100644
index 00000000..fe06c9fd
--- /dev/null
+++ b/workspace/tests/test_agent_card_well_known_path.py
@@ -0,0 +1,84 @@
+"""Pin the agent-card readiness probe to the SDK's canonical path.
+
+main.py's _send_initial_prompt() polls the local A2A server's
+well-known agent-card URL to know when it's safe to send the initial
+prompt as a self-message. Pre-fix the URL was hardcoded to the pre-1.x
+literal; a2a-sdk 1.x renamed the well-known path (the canonical value
+lives in `a2a.utils.constants.AGENT_CARD_WELL_KNOWN_PATH`), so the
+probe got 404 every attempt and silently fell through to "server not
+ready after 30s, skipping" — dropping every workspace's
+`initial_prompt` from config.yaml.
+
+The fix is to import the SDK's `AGENT_CARD_WELL_KNOWN_PATH` constant
+and use it directly in the probe URL. These tests pin the static
+invariants of that fix:
+
+  1. No hardcoded `/.well-known/agent.json` literal anywhere in
+     main.py (catches a future contributor reverting to a literal).
+  2. The probe URL fstring interpolates `AGENT_CARD_WELL_KNOWN_PATH`
+     (catches a "fix" that imports the constant for show but still
+     uses a literal in the actual GET).
+
+Note: we deliberately do not assert the constant's value or compare
+it against `create_agent_card_routes()` here. The runtime SDK is
+mocked in this directory's conftest for the executor-test path, so
+any test that imports the real `a2a.utils.constants` would either
+collide with the mock or require running in a separate pytest session.
+The two static invariants are sufficient: by always following whatever
+the SDK constant says, we travel through any rename automatically. The
+SDK's own contract that `create_agent_card_routes` mounts at the
+constant's value is the SDK's responsibility, not ours.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+WORKSPACE_ROOT = Path(__file__).resolve().parents[1]
+
+
+def test_main_uses_sdk_constant_for_agent_card_probe():
+    """No hardcoded `/.well-known/agent.json` literal anywhere in main.py.
+
+    The SDK constant (AGENT_CARD_WELL_KNOWN_PATH) is the single source
+    of truth — string-literal probes drift the moment the SDK renames.
+    """
+    main = (WORKSPACE_ROOT / "main.py").read_text()
+
+    bad_literal = "/.well-known/agent.json"
+    offenders = [
+        (lineno, line)
+        for lineno, line in enumerate(main.splitlines(), 1)
+        if bad_literal in line
+    ]
+    assert not offenders, (
+        f"Found pre-1.x literal {bad_literal!r} in main.py — must use "
+        f"the SDK's AGENT_CARD_WELL_KNOWN_PATH constant instead. "
+        f"Offending lines: {offenders}"
+    )
+
+    assert (
+        "AGENT_CARD_WELL_KNOWN_PATH" in main
+    ), "main.py must import a2a.utils.constants.AGENT_CARD_WELL_KNOWN_PATH"
+
+
+def test_probe_loop_uses_constant_in_url_format():
+    """Spot-check that the URL fstring in main.py interpolates the
+    constant, not a literal. Catches a future "fix" that imports the
+    constant for show but still uses a literal in the actual GET."""
+    main = (WORKSPACE_ROOT / "main.py").read_text()
+
+    # The probe pattern: `client.get(f"http://127.0.0.1:{port}{...}")`
+    # where `{...}` must be `{AGENT_CARD_WELL_KNOWN_PATH}`, not a
+    # hardcoded path.
+    pattern = re.compile(
+        r'client\.get\(f"http://127\.0\.0\.1:\{port\}\{(?P<expr>[^}]+)\}"\)'
+    )
+    matches = pattern.findall(main)
+    assert matches, "no readiness probe pattern found in main.py"
+    for expr in matches:
+        assert "AGENT_CARD_WELL_KNOWN_PATH" in expr, (
+            f"readiness probe URL uses {expr!r} instead of "
+            f"AGENT_CARD_WELL_KNOWN_PATH"
+        )