diff --git a/tier1/asl-indexes-1.md b/tier1/asl-indexes-1.md
index f0d7c7d..fe53f6d 100644
--- a/tier1/asl-indexes-1.md
+++ b/tier1/asl-indexes-1.md
@@ -76,6 +76,38 @@ Properties:
 
 This is the ASL/1-CORE-INDEX and is the only index that governs visibility.
 
+### 2.2 Structural Identity (SID)
+
+SID is the canonical identity of a derivation, not of bytes.
+
+```
+SID = H(ProgramRef || Inputs[] || ParamsRef || ExecProfile)
+```
+
+Notes:
+
+* `Inputs[]` order is canonical and stable.
+* `ParamsRef` is optional; absence must be encoded explicitly in the hash.
+* `ExecProfile` captures execution profile/versioning parameters (optional, but
+  presence/absence is part of the SID).
+
+### 2.2.1 SID Canonicalization (Normative)
+
+Implementations MUST canonicalize SID inputs as follows:
+
+1. **ProgramRef** is encoded as `ReferenceBytes` (`ENC/ASL1-CORE`).
+2. **Inputs[]** are ordered exactly as declared by the Program DAG inputs.
+3. **ParamsRef** is encoded as:
+   * `0x00` if absent, or
+   * `0x01 || ReferenceBytes` if present.
+4. **ExecProfile** is encoded as:
+   * `0x00` if absent, or
+   * `0x01 || ExecProfileBytes` if present.
+5. **SID hash input** is the concatenation of the above fields with no padding.
+
+`ExecProfileBytes` is an opaque, deterministic byte sequence defined by the
+execution environment. Any change in encoding or content MUST change SID.
+
 ### 2.2 Structural Index
 
 Purpose: map structural identity to a derivation DAG node.
@@ -90,7 +122,51 @@ Properties:
 * Does not imply materialization
 * May be in-memory or persisted
 
-### 2.3 Materialization Cache
+### 2.3 Derivation Index
+
+Purpose: map a materialized ArtifactKey to the set of known derivations that
+produce it.
+
+```
+ArtifactKey -> [DerivationRecord]
+```
+
+Where:
+
+```
+DerivationRecord = { SID, ProgramRef, InputRefs[], ParamsRef, ExecProfile }
+```
+
+Properties:
+
+* Authoritative for known derivations
+* Recomputable from replay + execution, so storage is optional
+* Enables dedup and semantic correlation across multiple derivations
+* Multiple SIDs MAY map to the same ArtifactKey
+
+#### 2.3.1 DerivationRecord Data Model (Normative)
+
+Canonical fields:
+
+```
+DerivationRecord {
+  sid: SID
+  program_ref: Reference
+  input_refs: Reference[]   // ordered as Program DAG inputs
+  params_ref: Optional<Reference>
+  exec_profile: Optional<ExecProfileBytes>
+}
+```
+
+Rules:
+
+* `input_refs` order MUST be preserved.
+* `params_ref` and `exec_profile` MUST use explicit presence markers as defined
+  in SID canonicalization.
+* Additional metadata MAY be stored but MUST NOT affect SID or canonical
+  equivalence.
+
+### 2.4 Materialization Cache
 
 Purpose: record previously materialized content for a structural identity.
 
@@ -112,6 +188,7 @@ Dependencies MUST follow this direction:
 
 ```
 Structural Index -> Materialization Cache -> Content Index
+Derivation Index -> Content Index
 ```
 
 Rules:
@@ -119,6 +196,7 @@ Rules:
 * The Content Index MUST NOT depend on the Structural Index.
 * The Structural Index MUST NOT depend on stored bytes.
 * The Materialization Cache MAY depend on both.
+* The Derivation Index MAY depend on the Content Index.
 
 ---
 
@@ -126,7 +204,10 @@ Rules:
 
 * PUT registers structure (if used), resolves to an ArtifactKey, and updates the Content Index.
 * GET consults only the Content Index and reads bytes from the store.
-* The Structural Index and Materialization Cache are optional optimizations for PUT.
+* The Structural Index, Derivation Index, and Materialization Cache are optional
+  optimizations for PUT.
+
+Note: versioning relationships are modeled in TGK, not in these indexes.
 
 ---
 
@@ -137,3 +218,9 @@ ASL/INDEXES/1 does not define:
 * Encodings for any index
 * Storage layout or sharding
 * Query operators or traversal semantics
+
+---
+
+## Changelog
+
+- 2026-01-18: Added SID canonicalization rules and DerivationRecord data model.