diff --git a/CHANGELOG.md b/CHANGELOG.md
index 83858d1..1e7a4be 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ Follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) and
 ### Changed
 
 - `validate` errors when a manifest uses a Simplicity program in a `tapleaf` or `simf_fn` compute, or a `programs` entry, without declaring `"requires": ["simplicity"]`. Previously only covenant `utxo_types` triggered this. Add `"simplicity"` to `requires` if `validate` now reports it.
+- `validate` errors when a manifest sets values in hooks (`on_resolved`, `on_pre_broadcast`, `on_post_broadcast`) without declaring `"requires": ["hooks"]`. Add `"hooks"` to `requires` if `validate` now reports it. `capabilities --supports` accepts `hooks`.
 - `validate` also checks each program file: that it exists, matches its hash, meets its compiler requirement, and declares the witnesses the manifest supplies.
 
 ## [0.3.0] - 2026-10-06
diff --git a/docs/decisions/0002-hooks-capability.md b/docs/decisions/0002-hooks-capability.md
new file mode 100644
index 0000000..d8aa1c5
--- /dev/null
+++ b/docs/decisions/0002-hooks-capability.md
@@ -0,0 +1,43 @@
+# 0002: Hooks are an optional capability
+
+- **Status:** accepted
+- **Date:** 2026-10-07
+
+## Context
+
+Hooks let a manifest set values while a transaction is being built: `on_resolved` on an
+input (typically to record an asset id an issuance input just created), and
+`on_pre_broadcast` / `on_post_broadcast` on an action.
+
+Neither of the existing browser-extension wallets (humid, apogee) runs hooks. Both refuse
+manifests that contain them. A wallet that doesn't implement hooks had no way to know in
+advance that a manifest needs them; it found out by parsing the manifest or partway
+through a run.
+
+`requires` already exists for this kind of question: it lists what a wallet must support,
+and a wallet without Simplicity refuses any manifest that requires `simplicity`.
+
+## Decision
+
+1. **`hooks` is a core capability**, alongside `simplicity`. A manifest that sets a value in
+   any hook must declare `"requires": ["hooks"]`. A hook block with an empty `set` does
+   nothing and needs nothing.
+2. **It is inferred and checked like `simplicity`.** `validate` reports a manifest that uses
+   hooks without declaring them, and a wallet that doesn't run hooks refuses a manifest
+   that requires them, before doing anything else.
+3. **`create_instance` is not a hook** and stays in the core format.
+4. **The two lending examples keep their hooks** and declare `hooks`, so they show the
+   capability in use. They are wire-compatible with the live simplicity-lending protocol,
+   and rewriting them without hooks would need re-checking that on a test network.
+
+## Consequences
+
+- `tx-manifest-wallet` runs hooks on every network, so its targets always provide `hooks`.
+  Another wallet reports what it supports with `capabilities --supports`, e.g.
+  `--supports simplicity` for one that compiles Simplicity but runs no hooks; the lending
+  examples then come back unsupported.
+- A manifest that used hooks without declaring them now fails `validate`. It was already
+  unrunnable by a wallet without hooks; now that's visible up front.
+- Unlike `simplicity`, `hooks` is a property of the wallet, not of the chain or the node.
+  The capability list is the residue the `chain` field can't settle, which includes
+  optional parts of the format as well as soft forks.
diff --git a/docs/format-changelog.md b/docs/format-changelog.md
index d079cb0..b41ae57 100644
--- a/docs/format-changelog.md
+++ b/docs/format-changelog.md
@@ -17,8 +17,9 @@ The JSON Schema for the current version is
 
 ## [0.3.1] - unreleased
 
-Additive, with one tightened rule (below): a `0.3.0` manifest is a valid `0.3.1` manifest
-unless it uses a Simplicity program without declaring `simplicity`.
+Additive, with two tightened rules (below): a `0.3.0` manifest is a valid `0.3.1` manifest
+unless it uses a Simplicity program without declaring `simplicity`, or a hook without
+declaring `hooks`.
 
 ### Added
 
@@ -36,6 +37,9 @@ unless it uses a Simplicity program without declaring `simplicity`.
     SimplicityHL's `simc` directive (`"0.7.1"` means `^0.7.1`). Priority: the reference or
     entry, then the manifest-wide value, then the file's own `simc` directive. The manifest
     can narrow the directive but not widen it.
+- **`hooks` capability** ([decision 0002](decisions/0002-hooks-capability.md)): hooks
+  (`on_resolved`, `on_pre_broadcast`, `on_post_broadcast`) are an optional part of the
+  format. A wallet may not implement them, and refuses manifests that require them.
 - **Pinned and unpinned manifests.** A program is pinned when it has a hash and a
   compiler-version requirement; a manifest is pinned when all its programs are. Wallets
   must refuse unpinned manifests and programs that don't match their hash. Tools may
@@ -56,6 +60,8 @@ unless it uses a Simplicity program without declaring `simplicity`.
   Each of these needs a compiler, and `requires` is how a wallet without one knows to
   refuse the manifest before trying it. A manifest that relied on the narrower rule was
   already unrunnable by such a wallet; it now fails validation instead.
+- A manifest that sets a value in any hook must declare `"requires": ["hooks"]`. A hook
+  block with an empty `set` needs nothing.
 
 ## [0.3.0] - 2026-10-06
 
diff --git a/docs/glossary.md b/docs/glossary.md
index cefa127..d3ae8cf 100644
--- a/docs/glossary.md
+++ b/docs/glossary.md
@@ -6,8 +6,10 @@ Terms as this repo uses them. Where a term is defined in more detail, the entry
 outputs it creates. Declared under `actions`, or under a contract template.
 
 **Capability.** Something a wallet must support to run a manifest, listed in the
-manifest's `requires` (e.g. `"simplicity"`). Third parties may define their own under a
-namespace (`ns::name`).
+manifest's `requires`. The format defines `simplicity` (the manifest uses a Simplicity
+program) and `hooks` (it sets values in hooks; see
+[decision 0002](decisions/0002-hooks-capability.md)). Third parties may define their own
+under a namespace (`ns::name`).
 
 **Compiler-version requirement.** The SimplicityHL versions a program is written for, as a
 semver requirement in the syntax of SimplicityHL's `simc` directive: `"0.7.1"` means
@@ -19,6 +21,10 @@ hex>"`. See [decision 0001](decisions/0001-program-pinning.md).
 **Contract template.** A reusable contract definition under `contract_templates`: typed
 fields plus the actions that operate on one instance of it.
 
+**Hook.** A block that sets values while a transaction is being built: `on_resolved` on an
+input, `on_pre_broadcast` / `on_post_broadcast` on an action. Optional for a wallet to
+support; a manifest using one requires `hooks`.
+
 **Instance.** One deployed contract made from a template: the values its fields were given
 when it was created.
 
diff --git a/examples/lending_v2/txmanifest.json b/examples/lending_v2/txmanifest.json
index ca07149..4a4d997 100644
--- a/examples/lending_v2/txmanifest.json
+++ b/examples/lending_v2/txmanifest.json
@@ -2,7 +2,7 @@
   "$schema": "../../schema/txmanifest.schema.json",
   "manifest_version": "0.3.1",
   "protocol": "simplicity-lending",
-  "requires": ["simplicity"],
+  "requires": ["simplicity", "hooks"],
   "simplicity_hl": { "version": "0.7.1", "debug_symbols": true },
   "description": "P2P collateralised lending on Liquid, wire-compatible with the simplicity-lending reference implementation (github BlockstreamResearch/simplicity-lending, `smplx-sdk` covenants). This 'v2' manifest reproduces that protocol's exact on-chain transaction layout so offers created with tx-manifest-wallet are discoverable and settleable by simplicity-lending's own CLI / indexer / web app, and vice-versa. Key differences from the standalone 'lending' example: (1) the principal payout AND the borrower NFT both go to the borrower's plain wallet address (an explicit v0 P2WPKH), committed into the covenant as sha256(scriptPubKey) — there is no p2pk covenant and no separate claim step; (2) the borrower NFT is held in the borrower's wallet during the active loan and spent with an ordinary signature on repayment; (3) NFT ordering is first-params, second-params, borrower, lender; (4) the creation OP_RETURN carries borrower_pubkey || principal_asset_id (64 bytes, internal asset order) for indexer discovery; (5) a single AMOUNTS_DECIMALS drives the bit-packed parameter NFTs, matching the reference wallet.",
   "utxo_types": {
diff --git a/examples/lending_v3/txmanifest.json b/examples/lending_v3/txmanifest.json
index cdc72ec..6adb93b 100644
--- a/examples/lending_v3/txmanifest.json
+++ b/examples/lending_v3/txmanifest.json
@@ -3,7 +3,7 @@
   "$comment": "lending_v3 targets the DEPLOYED simplicity-lending indexer (odev). Interop constants are fixed: factory params (2,0), NUMS key, protocol-fee keeper 38fca2d9…, and the program-id tags below. Change a covenant .simf and you must recompute the matching *_PROGRAM_ID + covenant address.",
   "manifest_version": "0.3.1",
   "protocol": "simplicity-lending",
-  "requires": ["simplicity"],
+  "requires": ["simplicity", "hooks"],
   "simplicity_hl": { "version": "0.7.1", "debug_symbols": true },
   "description": "lending_v3 — the redesigned 'issuance factory' lending protocol, wire-compatible with the deployed simplicity-lending indexer/site (odev branch). Phase 3a models factory creation: a persistent issuance_factory covenant plus the wallet-held auth NFT, from which many lending offers are later minted.",
   "utxo_types": {
diff --git a/schema/txmanifest.schema.json b/schema/txmanifest.schema.json
index ddba907..02a7ad1 100644
--- a/schema/txmanifest.schema.json
+++ b/schema/txmanifest.schema.json
@@ -143,7 +143,8 @@
         {
           "description": "A capability defined by this format.",
           "enum": [
-            "simplicity"
+            "simplicity",
+            "hooks"
           ]
         },
         {
diff --git a/txmanifest_lib/src/chain.rs b/txmanifest_lib/src/chain.rs
index a6bce03..f3ac632 100644
--- a/txmanifest_lib/src/chain.rs
+++ b/txmanifest_lib/src/chain.rs
@@ -14,9 +14,10 @@
 //!   says there are no native assets, and a manifest repeating that in a feature list
 //!   would only create a second place to disagree.
 //! - **[`Capability`]** — what a *manifest* must state because the chain alone does not
-//!   settle it. Today that is exactly one thing, [`Capability::SIMPLICITY`], because
-//!   Simplicity is a soft fork on Bitcoin rather than a property of it. Plus whatever
-//!   third parties define under their own namespace.
+//!   settle it. Today that is two things: [`Capability::SIMPLICITY`], because Simplicity
+//!   is a soft fork on Bitcoin rather than a property of it, and [`Capability::HOOKS`],
+//!   because running hooks is a property of the wallet rather than of any chain. Plus
+//!   whatever third parties define under their own namespace.
 //! - **[`Activation`]** — what the specific node being talked to actually provides. This
 //!   is configuration, not discovery.
 //!
@@ -29,8 +30,9 @@
 //! using issuance on Bitcoin is still an error — they just read the chain instead of a
 //! restatement of it. See [`ChainFamily::has_native_assets`] and its neighbours.
 //!
-//! What is left in [`Capability`] is the residue that genuinely cannot be inferred: a soft
-//! fork's activation state, and features this crate has never heard of.
+//! What is left in [`Capability`] is the residue the chain cannot settle: a soft fork's
+//! activation state, an optional part of the format a wallet may not implement (hooks),
+//! and features this crate has never heard of.
 //!
 //! # Namespaces
 //!
@@ -251,15 +253,22 @@ pub enum CoreCapability {
     /// Covenant `utxo_types` backed by SimplicityHL programs, spent through a Simplicity
     /// tapleaf the validator executes.
     ///
-    /// The one core capability, because it is the one thing the `chain` field does not
-    /// settle. On Elements it is live. On Bitcoin it is a proposed soft fork (BINANA
+    /// Not settled by the `chain` field. On Elements it is live. On Bitcoin it is a proposed soft fork (BINANA
     /// 2026-0003) — implemented in the C library, but not activated on mainnet, testnet,
     /// or the default signet — so whether a given Bitcoin node honours it is a property of
     /// that node, not of Bitcoin.
     ///
-    /// A manifest that declares no `utxo_types` with a `script` does not need this, and
-    /// can target a stock Bitcoin node.
+    /// A manifest that uses no Simplicity program does not need this, and can target a
+    /// stock Bitcoin node.
     Simplicity,
+    /// Hooks: `on_resolved` on an input, `on_pre_broadcast` / `on_post_broadcast` on an
+    /// action, which set values while a transaction is being built.
+    ///
+    /// An optional part of the format rather than a property of any chain: the existing
+    /// browser-extension wallets run none of them, and a wallet that doesn't must be able to
+    /// refuse a manifest that relies on them before it starts, not partway through.
+    /// `create_instance` is not a hook and needs no capability.
+    Hooks,
 }
 
 impl CoreCapability {
@@ -267,6 +276,7 @@ impl CoreCapability {
     pub fn as_str(self) -> &'static str {
         match self {
             CoreCapability::Simplicity => "simplicity",
+            CoreCapability::Hooks => "hooks",
         }
     }
 }
@@ -275,8 +285,11 @@ impl Capability {
     /// The Simplicity capability, spelled out for call sites.
     pub const SIMPLICITY: Capability = Capability::Core(CoreCapability::Simplicity);
 
+    /// The hooks capability, spelled out for call sites.
+    pub const HOOKS: Capability = Capability::Core(CoreCapability::Hooks);
+
     /// Every core capability this build defines.
-    pub const CORE: [CoreCapability; 1] = [CoreCapability::Simplicity];
+    pub const CORE: [CoreCapability; 2] = [CoreCapability::Simplicity, CoreCapability::Hooks];
 
     /// Build a namespaced capability, validating both halves.
     pub fn namespaced(
@@ -315,6 +328,11 @@ impl Capability {
                  activated"
                     .to_string()
             }
+            Capability::Core(CoreCapability::Hooks) => {
+                "the manifest sets values in on_resolved / on_pre_broadcast / \
+                 on_post_broadcast hooks, which this wallet does not run"
+                    .to_string()
+            }
             Capability::Namespaced { namespace, .. } => format!(
                 "defined by '{namespace}', not by this format; a target provides it only by \
                  listing it as an extension"
@@ -553,6 +571,8 @@ impl Network {
         if simplicity_live {
             caps.insert(Capability::SIMPLICITY);
         }
+        // Hooks are run by this wallet, not by the node, so every network has them.
+        caps.insert(Capability::HOOKS);
         caps
     }
 
diff --git a/txmanifest_lib/src/manifest.rs b/txmanifest_lib/src/manifest.rs
index fd85472..4eef8a7 100644
--- a/txmanifest_lib/src/manifest.rs
+++ b/txmanifest_lib/src/manifest.rs
@@ -1669,7 +1669,8 @@ impl Manifest {
     /// `requires` claims.
     ///
     /// Only the residue that [`Manifest::chain`] does not settle, which today means: does
-    /// anything need a Simplicity compiler. That is any `utxo_type` with a `script`, and
+    /// anything set a value in a hook (`hooks`), and does anything need a Simplicity
+    /// compiler. That is any `utxo_type` with a `script`, and
     /// any other program reference — a `tapleaf` or `simf_fn` compute, or a `programs`
     /// entry — since computing a tapleaf hash compiles the program just as deriving a
     /// covenant address does. A wallet without a compiler relies on this to refuse such a
@@ -1687,9 +1688,27 @@ impl Manifest {
         if uses_covenants || !program_refs.is_empty() {
             caps.insert(Capability::SIMPLICITY);
         }
+        if self.uses_hooks() {
+            caps.insert(Capability::HOOKS);
+        }
         caps
     }
 
+    /// Whether any action or input sets a value in a hook. A hook block with an empty
+    /// `set` does nothing, so it does not count.
+    fn uses_hooks(&self) -> bool {
+        let used = |h: &Option<HookBlock>| h.as_ref().is_some_and(|h| !h.set.is_empty());
+        let mut actions = self.actions.values().collect::<Vec<_>>();
+        for t in self.contract_templates.iter().flat_map(|t| t.values()) {
+            actions.extend(t.actions.values());
+        }
+        actions.into_iter().any(|a| {
+            used(&a.on_pre_broadcast)
+                || used(&a.on_post_broadcast)
+                || a.inputs.iter().flatten().any(|i| used(&i.on_resolved))
+        })
+    }
+
     /// Places where this manifest uses something its declared [`Manifest::chain`] does not
     /// have.
     ///
diff --git a/txmanifest_lib/src/validate.rs b/txmanifest_lib/src/validate.rs
index 9966f25..0cbde89 100644
--- a/txmanifest_lib/src/validate.rs
+++ b/txmanifest_lib/src/validate.rs
@@ -1258,6 +1258,48 @@ mod tests {
         }
     }
 
+    /// Hooks are optional for a wallet to implement, so a manifest that sets a value in one
+    /// must say so: an undeclared hook is the same kind of error as undeclared Simplicity.
+    #[test]
+    fn a_manifest_using_hooks_must_declare_them() {
+        let manifest = |requires: &str, hook: &str| {
+            Manifest::from_json_str(&format!(
+                r#"{{ "manifest_version": "0.3.1", "protocol": "t", "chain": "elements",
+                      "requires": {requires},
+                      "actions": {{ "A": {{
+                        "params": {{ "B": {{ "type": "u64", "compute": {{ "type": "hook" }} }} }},
+                        {hook} }} }} }}"#
+            ))
+            .expect("manifest should parse")
+        };
+        for hook in [
+            r#""on_pre_broadcast": { "set": { "params.B": "1" } }"#,
+            r#""on_post_broadcast": { "set": { "params.B": "1" } }"#,
+            r#""inputs": [ { "id": "i", "utxo_source": "wallet",
+                             "on_resolved": { "set": { "params.B": "1" } } } ]"#,
+        ] {
+            let report = validate(&manifest("[]", hook));
+            assert!(
+                messages(&report).contains("uses 'hooks' but does not declare it"),
+                "{hook}: {}",
+                messages(&report)
+            );
+            let report = validate(&manifest(r#"["hooks"]"#, hook));
+            assert!(
+                !messages(&report).contains("'hooks'"),
+                "{hook}: {}",
+                messages(&report)
+            );
+        }
+        // An empty hook block sets nothing, so it needs nothing.
+        let report = validate(&manifest("[]", r#""on_pre_broadcast": { "set": {} }"#));
+        assert!(
+            !messages(&report).contains("'hooks'"),
+            "{}",
+            messages(&report)
+        );
+    }
+
     /// Simplicity is legal to *declare* on Bitcoin — it is a specified soft fork, and
     /// whether a given node honours it is settled by the target, not by `validate`.
     #[test]
@@ -1805,6 +1847,7 @@ mod tests {
             &serde_json::json!({
                 "manifest_version": "0.3.0",
                 "protocol": "test",
+                "requires": ["hooks"],
                 "actions": { "A": { "params": params, "on_pre_broadcast": { "set": set } } }
             })
             .to_string(),