docs: define automation compatibility policy

README.md

@@ -624,6 +624,11 @@ capability, or creating/registering a missing resource.
 
 ## CI, Security, And Releases
 
+The automation-facing compatibility policy is documented in
+[`docs/compatibility.md`](docs/compatibility.md). It defines the intended
+stability rules for CLI commands, `--json` output, the local control JSONL
+protocol, Iroh peer wire protocols, SQLite metadata, and signed operation logs.
+
 GitHub Actions workflows live under `.github/workflows/`:
 
 - `ci.yml` runs formatting, clippy, docs, `cargo check`, and workspace tests on

crates/geth-control/src/lib.rs

@@ -3204,4 +3204,23 @@ mod tests {
             response
         );
     }
+
+    #[test]
+    fn malformed_and_unknown_protocol_inputs_fail() {
+        for input in [
+            "{",
+            "{}",
+            r#"{"type":"unknown"}"#,
+            r#"{"type":"status","extra":}"#,
+        ] {
+            assert!(decode_request(input).is_err());
+            assert!(decode_response(input).is_err());
+            assert!(decode_peer_request(input).is_err());
+            assert!(decode_peer_response(input).is_err());
+            assert!(decode_pipe_wire_request(input).is_err());
+            assert!(decode_pipe_wire_response(input).is_err());
+            assert!(decode_overlay_wire_request(input).is_err());
+            assert!(decode_overlay_wire_response(input).is_err());
+        }
+    }
 }

docs/compatibility.md

@@ -0,0 +1,105 @@
+# Compatibility Policy
+
+`geth` has not shipped a stable release yet. Until the first deployment tag,
+contracts may still change, but changes must be deliberate, documented in this
+file, and reflected in `docs/production-readiness-roadmap.md`.
+
+After the first deployment tag, compatibility is defined by the surfaces below.
+
+## CLI Commands
+
+Commands documented in `README.md` are the automation-facing CLI surface.
+Existing command names, positional argument meanings, flag names, and default
+behavior should remain compatible within a major release.
+
+Backward-compatible CLI changes include:
+
+- Adding optional flags with defaults that preserve previous behavior.
+- Adding new subcommands.
+- Adding fields to human-readable output when `--json` is not requested.
+- Improving error messages while preserving the error kind and recovery path.
+
+Breaking CLI changes include:
+
+- Renaming or removing a documented command or flag.
+- Changing the meaning of an existing argument or default.
+- Changing exit success for the same observable result.
+- Requiring a daemon, network path, privileged service, or host mutation where a
+  documented local command did not previously require one.
+
+Prototype commands must be called out in `README.md` or feature docs with an
+explicit migration expectation before scripts depend on them.
+
+## JSON Output
+
+`--json` output is the preferred script contract. For stable commands, the
+top-level response variant name and existing field names keep their meaning
+within a major release.
+
+Backward-compatible JSON changes include:
+
+- Adding nullable or optional fields.
+- Adding enum variants for new commands or new explicit states.
+- Adding elements to arrays where ordering was not documented as stable.
+
+Breaking JSON changes include:
+
+- Removing or renaming fields.
+- Changing field type, unit, or identifier format.
+- Reusing a field for a different semantic meaning.
+- Changing a documented stable ordering.
+
+Automation should ignore unknown fields. Tests that assert JSON fixtures should
+cover the subset intended as stable rather than every incidental field.
+
+## Local Control JSONL
+
+The local daemon control socket uses newline-delimited JSON request and response
+messages from `geth-control`. This protocol is local-only and not a remote trust
+boundary, but local automation may still rely on it.
+
+Stable local-control variants follow the same compatibility rules as `--json`
+output. Unknown variants, malformed JSON, and messages without a trailing
+newline must fail without daemon state mutation. Local control must not become a
+remote transport.
+
+## Peer Wire Protocols
+
+Node-to-node geth communication is Iroh-only. Peer protocols are grouped by
+explicit ALPNs:
+
+- `/geth/control/1` for authenticated peer-control requests.
+- `/geth/pipe/1` for authorized pipe byte streams.
+- `/geth/overlay/1` for authorized overlay packets.
+
+Within a major release, existing peer request and response variants should keep
+their field names, field meanings, nonce behavior, peer-card validation, and
+endpoint-binding validation. New variants must use explicit capabilities and
+resource IDs. Discovery data remains untrusted and never grants capabilities.
+
+Breaking peer-wire changes require a version bump, migration note, and tests
+showing old messages fail closed rather than being interpreted as a different
+operation.
+
+## SQLite Metadata
+
+The metadata database is product state. Schema changes must be represented as
+ordered migrations before deployment. Fresh opens and repeated opens must be
+idempotent, and multi-table writes that represent one logical operation should
+commit atomically.
+
+Stable table columns may be added through migrations. Removing or repurposing a
+column requires a migration note and a recovery path. SQLite state is an
+implementation detail for scripts unless a table is explicitly documented as
+stable.
+
+## Signed Operations
+
+Signed keychain, auth, SSH certificate, and SSH revocation payloads are durable
+compatibility surfaces. Signed payloads must use deterministic canonical
+encoding with explicit namespaces and versions. A verifier must never treat
+arbitrary JSON serialization as signed canonical data.
+
+Changing a signed payload shape, namespace, or verification rule requires a new
+versioned namespace or a documented migration that preserves validation of
+existing signed history.

docs/production-readiness-roadmap.md

@@ -86,13 +86,13 @@ behavior.
 Goal: make command, JSON, and protocol contracts explicit enough for scripts
 and downstream projects.
 
-- `[ ]` Define compatibility policy.
+- `[x]` Define compatibility policy.
   Acceptance criteria:
-  - `[ ]` CLI command compatibility is documented.
+  - `[x]` CLI command compatibility is documented.
-  - `[ ]` `--json` output compatibility is documented.
+  - `[x]` `--json` output compatibility is documented.
-  - `[ ]` Local control JSONL compatibility is documented.
+  - `[x]` Local control JSONL compatibility is documented.
-  - `[ ]` Peer wire protocol compatibility is documented.
+  - `[x]` Peer wire protocol compatibility is documented.
-  - `[ ]` SQLite and signed-operation compatibility are documented.
+  - `[x]` SQLite and signed-operation compatibility are documented.
 
 - `[ ]` Add golden JSON tests.
   Acceptance criteria:
@@ -101,13 +101,13 @@ and downstream projects.
     failures.
   - `[ ]` Fixture updates require intentional review.
 
-- `[ ]` Expand protocol roundtrip tests.
+- `[~]` Expand protocol roundtrip tests.
   Acceptance criteria:
   - `[ ]` Every `ControlRequest` and `ControlResponse` variant roundtrips.
   - `[ ]` Every `PeerControlRequest` and `PeerControlResponse` variant
     roundtrips.
   - `[ ]` Pipe and overlay wire protocol variants roundtrip.
-  - `[ ]` Unknown or malformed protocol inputs fail safely.
+  - `[x]` Unknown or malformed protocol inputs fail safely.
 
 - `[ ]` Classify command stability.
   Acceptance criteria: