Slack error message_not_found: No message at that timestamp

You asked chat.update or chat.delete to operate on a message identified by (channel, ts), and Slack has no such message. Both methods document the description as "No message exists with the requested timestamp." The ts value is the message's identity — a string like "1503435956.000247" returned by the original chat.postMessage — and it only means something in combination with the exact channel it was posted to.

Three failure shapes dominate. First, precision loss: ts looks like a float, and languages that parse it as one quietly mangle the trailing digits; 1503435956.000247 stored as a double and re-serialized may not round-trip, and the mutated value matches nothing. Keep it a string, always. Second, channel mismatch: updating with the right ts but a different channel (the message was posted to a DM but the update targets the channel, or vice versa) — the pair must match the original send. Third, the message is gone: deleted by a user or admin, or removed by retention policy, in which case no identifier will ever find it again.

The error is deterministic per (channel, ts) pair; retries are useless. When it appears in an edit/undo feature, degrade gracefully — the underlying message no longer exists, so the feature should reflect that instead of erroring.

POST /api/chat.update
{"channel": "C0123ABCDEF", "ts": "1503435956.0002", "text": "edit"}

HTTP 200
{"ok": false, "error": "message_not_found"}

Persist the full chat.postMessage response — at minimum channel and ts together, as strings — the moment a send succeeds, and treat that pair as the message's immutable address. Schema-check that no numeric type ever touches ts.