diff --git a/registry/hash.go b/registry/hash.go
index ab23fa87..83b28102 100644
--- a/registry/hash.go
+++ b/registry/hash.go
@@ -37,6 +37,7 @@ func ComputeFunctionHash(f *Function) string {
 	fmt.Fprintf(h, "|%s", marshalStrings(f.Variant))
 	fmt.Fprintf(h, "|%s|%s|%s", f.Notes, f.Documentation, f.Code)
 	fmt.Fprintf(h, "|%s|%s|%s", f.SourceRepo, f.SourceLicense, f.SourceFile)
+	fmt.Fprintf(h, "|%s", f.ParamsSchema)
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
 
diff --git a/registry/indexer.go b/registry/indexer.go
index a38422a4..6970a098 100644
--- a/registry/indexer.go
+++ b/registry/indexer.go
@@ -238,6 +238,8 @@ func Index(db *DB, root string) (*IndexResult, error) {
 	}
 
 	// Post-insert: warn about file_path entries that don't exist on disk
+	// and functions missing params_schema
+	missingParams := 0
 	for _, f := range functions {
 		if f.FilePath != "" {
 			abs := filepath.Join(root, f.FilePath)
@@ -251,6 +253,12 @@ func Index(db *DB, root string) (*IndexResult, error) {
 				result.Warnings = append(result.Warnings, fmt.Sprintf("%s: test_file_path %q not found", f.ID, f.TestFilePath))
 			}
 		}
+		if f.ParamsSchema == "" {
+			missingParams++
+		}
+	}
+	if missingParams > 0 {
+		result.Warnings = append(result.Warnings, fmt.Sprintf("%d functions missing params_schema (run 'fn check params' to list)", missingParams))
 	}
 	for _, t := range types {
 		if t.FilePath != "" {
diff --git a/registry/migrations/009_params_schema.sql b/registry/migrations/009_params_schema.sql
new file mode 100644
index 00000000..97aa53b5
--- /dev/null
+++ b/registry/migrations/009_params_schema.sql
@@ -0,0 +1,51 @@
+-- Add params_schema to functions: JSON with semantic descriptions of inputs/outputs.
+-- Format: {"params":[{"name":"x","desc":"..."}],"output":"..."}
+-- Enables agent reasoning about function composability.
+
+ALTER TABLE functions ADD COLUMN params_schema TEXT NOT NULL DEFAULT '';
+
+-- Rebuild FTS for functions: add params_schema
+DROP TRIGGER IF EXISTS functions_ai;
+DROP TRIGGER IF EXISTS functions_ad;
+DROP TRIGGER IF EXISTS functions_au;
+
+INSERT INTO functions_fts(functions_fts) VALUES('rebuild');
+DROP TABLE IF EXISTS functions_fts;
+
+CREATE VIRTUAL TABLE functions_fts USING fts5(
+    id,
+    name,
+    description,
+    tags,
+    signature,
+    domain,
+    example,
+    notes,
+    documentation,
+    code,
+    params_schema,
+    content='functions',
+    content_rowid='rowid'
+);
+
+-- Populate FTS from existing data
+INSERT INTO functions_fts(rowid, id, name, description, tags, signature, domain, example, notes, documentation, code, params_schema)
+SELECT rowid, id, name, description, tags, signature, domain, example, notes, documentation, code, params_schema
+FROM functions;
+
+CREATE TRIGGER functions_ai AFTER INSERT ON functions BEGIN
+    INSERT INTO functions_fts(rowid, id, name, description, tags, signature, domain, example, notes, documentation, code, params_schema)
+    VALUES (new.rowid, new.id, new.name, new.description, new.tags, new.signature, new.domain, new.example, new.notes, new.documentation, new.code, new.params_schema);
+END;
+
+CREATE TRIGGER functions_ad AFTER DELETE ON functions BEGIN
+    INSERT INTO functions_fts(functions_fts, rowid, id, name, description, tags, signature, domain, example, notes, documentation, code, params_schema)
+    VALUES ('delete', old.rowid, old.id, old.name, old.description, old.tags, old.signature, old.domain, old.example, old.notes, old.documentation, old.code, old.params_schema);
+END;
+
+CREATE TRIGGER functions_au AFTER UPDATE ON functions BEGIN
+    INSERT INTO functions_fts(functions_fts, rowid, id, name, description, tags, signature, domain, example, notes, documentation, code, params_schema)
+    VALUES ('delete', old.rowid, old.id, old.name, old.description, old.tags, old.signature, old.domain, old.example, old.notes, old.documentation, old.code, old.params_schema);
+    INSERT INTO functions_fts(rowid, id, name, description, tags, signature, domain, example, notes, documentation, code, params_schema)
+    VALUES (new.rowid, new.id, new.name, new.description, new.tags, new.signature, new.domain, new.example, new.notes, new.documentation, new.code, new.params_schema);
+END;
diff --git a/registry/models.go b/registry/models.go
index 0f1eae68..a6c4c038 100644
--- a/registry/models.go
+++ b/registry/models.go
@@ -50,6 +50,7 @@ type Function struct {
 	Notes           string   `json:"notes"`
 	Documentation   string   `json:"documentation"`
 	Code            string   `json:"code"`
+	ParamsSchema    string   `json:"params_schema"`
 	Tested          bool     `json:"tested"`
 	Tests           []string `json:"tests"`
 	TestFilePath    string   `json:"test_file_path"`
diff --git a/registry/parser.go b/registry/parser.go
index 50a4f862..8f46526e 100644
--- a/registry/parser.go
+++ b/registry/parser.go
@@ -2,6 +2,7 @@ package registry
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -32,6 +33,10 @@ type rawFunction struct {
 	TestFilePath    string    `yaml:"test_file_path"`
 	FilePath        string    `yaml:"file_path"`
 
+	// Params schema
+	Params []rawParam `yaml:"params"`
+	Output string     `yaml:"output"`
+
 	// Source attribution
 	SourceRepo    string `yaml:"source_repo"`
 	SourceLicense string `yaml:"source_license"`
@@ -45,6 +50,12 @@ type rawFunction struct {
 	Variant   []string  `yaml:"variant"`
 }
 
+// rawParam describes a function parameter with semantic meaning.
+type rawParam struct {
+	Name string `yaml:"name" json:"name"`
+	Desc string `yaml:"desc" json:"desc"`
+}
+
 // rawType mirrors the YAML frontmatter of a type .md file.
 type rawType struct {
 	Name        string   `yaml:"name"`
@@ -175,6 +186,17 @@ func ParseFunctionMD(path string, root string) (*Function, error) {
 		SourceFile:      raw.SourceFile,
 	}
 
+	// Serialize params + output to JSON for params_schema column
+	if len(raw.Params) > 0 || raw.Output != "" {
+		schema := struct {
+			Params []rawParam `json:"params,omitempty"`
+			Output string     `json:"output,omitempty"`
+		}{Params: raw.Params, Output: raw.Output}
+		if b, err := json.Marshal(schema); err == nil {
+			f.ParamsSchema = string(b)
+		}
+	}
+
 	if root != "" && raw.FilePath != "" {
 		codePath := filepath.Join(root, raw.FilePath)
 		if codeData, err := os.ReadFile(codePath); err == nil {
diff --git a/registry/store.go b/registry/store.go
index b2d8d102..246097ba 100644
--- a/registry/store.go
+++ b/registry/store.go
@@ -86,7 +86,8 @@ func (db *DB) InsertFunction(f *Function) error {
 			tests, test_file_path, file_path, content_hash, created_at, updated_at,
 			props, emits, has_state, framework, variant,
 			notes, documentation, code,
-			source_repo, source_license, source_file
+			source_repo, source_license, source_file,
+			params_schema
 		) VALUES (
 			?, ?, ?, ?, ?, ?, ?, ?,
 			?, ?, ?, ?, ?,
@@ -94,7 +95,8 @@ func (db *DB) InsertFunction(f *Function) error {
 			?, ?, ?, ?, ?, ?,
 			?, ?, ?, ?, ?,
 			?, ?, ?,
-			?, ?, ?
+			?, ?, ?,
+			?
 		)`,
 		f.ID, f.Name, string(f.Kind), f.Lang, f.Domain, f.Version, string(f.Purity), f.Signature,
 		f.Description, marshalStrings(f.Tags), marshalStrings(f.UsesFunctions), marshalStrings(f.UsesTypes), marshalStrings(f.Returns),
@@ -103,6 +105,7 @@ func (db *DB) InsertFunction(f *Function) error {
 		marshalProps(f.Props), marshalStrings(f.Emits), hasState, f.Framework, marshalStrings(f.Variant),
 		f.Notes, f.Documentation, f.Code,
 		f.SourceRepo, f.SourceLicense, f.SourceFile,
+		f.ParamsSchema,
 	)
 	return err
 }
@@ -559,6 +562,7 @@ func scanFunctions(rows interface{ Next() bool; Scan(...any) error }) ([]Functio
 			&propsJSON, &emitsJSON, &hasState, &f.Framework, &variantJSON,
 			&f.Notes, &f.Documentation, &f.Code, &f.ContentHash,
 			&f.SourceRepo, &f.SourceLicense, &f.SourceFile,
+			&f.ParamsSchema,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("scanning function: %w", err)