diff --git a/README.md b/README.md
index ba8e6ae..62fdb53 100644
--- a/README.md
+++ b/README.md
@@ -49,6 +49,10 @@ bin/gsp.exe
 .\bin\gsp.exe index --root examples\lottery --out .gsp\index.json
 .\bin\gsp.exe flatten page.lottery.main --root examples\lottery --depth -1 --out .gsp\flattened.json
 .\bin\gsp.exe pack page.lottery.main --root examples\lottery --depth -1 --budget 12000 --out .gsp\context-pack.json
+.\bin\gsp.exe pack page.lottery.main --root examples\lottery --for implement --stage implement --format md --out .gsp\context-pack.md
+.\bin\gsp.exe impact feedback.positive --root examples\lottery --format md --out .gsp\impact.md
+.\bin\gsp.exe impact feedback.positive --root examples\lottery --format canvas --out .gsp\impact.canvas
+.\bin\gsp.exe message validate messages\implement-page.gspmsg --root examples\lottery --out .gsp\message-report.json
 .\bin\gsp.exe graph page.lottery.main --root examples\lottery --format mermaid --out .gsp\graph.mmd
 .\bin\gsp.exe graph page.lottery.main --root examples\lottery --format md --out .gsp\graph.md
 .\bin\gsp.exe graph page.lottery.main --root examples\lottery --format canvas --out .gsp\graph.canvas
@@ -63,6 +67,10 @@ bin/gsp.exe
 .gsp/index.json
 .gsp/flattened.json
 .gsp/context-pack.json
+.gsp/context-pack.md
+.gsp/impact.md
+.gsp/impact.canvas
+.gsp/message-report.json
 .gsp/graph.mmd
 .gsp/graph.md
 .gsp/graph.canvas
@@ -77,8 +85,10 @@ bin/gsp.exe
 | `specs/versions/0.1/README.md` | GSP 0.1 语言说明。 |
 | `specs/versions/0.1/gsp.schema.json` | GSP 0.1 核心字段 schema。 |
 | `specs/versions/0.1/gsp.manifest.schema.json` | GSP 0.1 工程 manifest schema。 |
+| `specs/versions/0.1/gsp.message.schema.json` | GSP 0.1 agent 通信消息 schema。 |
 | `specs/versions/0.1/commands.md` | GSP 0.1 命令规范。 |
 | `specs/versions/0.1/ai-usage.md` | GSP 0.1 AI 使用规则。 |
+| `specs/versions/0.1/message.md` | GSP Message 说明。 |
 | `toolkit/README.md` | GSP Toolkit 命令与实现说明。 |
 
 ## GSP 工程结构
@@ -144,6 +154,9 @@ gsp completion install powershell
 gsp validate
 gsp index
 gsp flatten <id>
+gsp pack <id> --for implement --format md
+gsp impact <id> --format md
+gsp message validate message.gspmsg
 gsp graph <id>
 gsp stage-check --stage implement
 ```
diff --git a/examples/lottery/README.md b/examples/lottery/README.md
index 5b51a95..c4fc592 100644
--- a/examples/lottery/README.md
+++ b/examples/lottery/README.md
@@ -24,6 +24,9 @@ page.lottery.main
 ```powershell
 .\bin\gsp.exe validate --root examples\lottery
 .\bin\gsp.exe flatten page.lottery.main --root examples\lottery --depth -1
+.\bin\gsp.exe pack page.lottery.main --root examples\lottery --for implement --format md
+.\bin\gsp.exe impact feedback.positive --root examples\lottery --format md
+.\bin\gsp.exe message validate messages\implement-page.gspmsg --root examples\lottery
 .\bin\gsp.exe graph page.lottery.main --root examples\lottery --format mermaid
 .\bin\gsp.exe graph page.lottery.main --root examples\lottery --format md --out .gsp\lottery-graph.md
 .\bin\gsp.exe graph page.lottery.main --root examples\lottery --format canvas --out .gsp\lottery-graph.canvas
diff --git a/examples/lottery/messages/implement-page.gspmsg b/examples/lottery/messages/implement-page.gspmsg
new file mode 100644
index 0000000..d48875c
--- /dev/null
+++ b/examples/lottery/messages/implement-page.gspmsg
@@ -0,0 +1,14 @@
+gspMessageVersion: 0.1
+id: msg.implement.lottery_page
+from: planner
+to: implementer
+intent: implement
+entry: page.lottery.main
+stage: implement
+requires:
+  - ui.button.reward_primary
+  - feedback.positive
+contextPack:
+  mode: implement
+  depth: -1
+  budget: 12000
diff --git a/specs/versions/0.1/README.md b/specs/versions/0.1/README.md
index 8c4ee65..e211923 100644
--- a/specs/versions/0.1/README.md
+++ b/specs/versions/0.1/README.md
@@ -13,8 +13,10 @@ README 面向准备使用 GSP 语言的人类和 AI。它说明 GSP 的用途、
 | `README.md` | GSP 语言使用前说明。给人类和 AI 阅读。 |
 | `gsp.schema.json` | GSP 第一版核心字段规范。使用 JSON Schema 表达，便于 AI、工具、编译器和实现模块识别。 |
 | `gsp.manifest.schema.json` | GSP 工程 manifest 字段规范。 |
+| `gsp.message.schema.json` | GSP agent 通信消息字段规范。 |
 | `commands.md` | GSP Toolkit 命令规范。 |
 | `ai-usage.md` | GSP 项目 AI 使用规则。 |
+| `message.md` | GSP Message 说明。 |
 
 ## GSP 是什么
 
diff --git a/specs/versions/0.1/ai-usage.md b/specs/versions/0.1/ai-usage.md
index 9c8b12b..ebb02f1 100644
--- a/specs/versions/0.1/ai-usage.md
+++ b/specs/versions/0.1/ai-usage.md
@@ -16,4 +16,6 @@
 - Use `gsp trace <id>` to inspect relations.
 - Use `gsp flatten <id>` before implementation or task splitting.
 - Use `gsp pack <id>` when a compact AI context is needed.
+- Use `gsp impact <id>` before changing shared GSP units.
+- Use `gsp message validate <file>` for agent communication messages.
 - Use `gsp stage-check --stage <stage>` before stage handoff.
diff --git a/specs/versions/0.1/commands.md b/specs/versions/0.1/commands.md
index 8d2942a..3364070 100644
--- a/specs/versions/0.1/commands.md
+++ b/specs/versions/0.1/commands.md
@@ -100,9 +100,55 @@ gsp flatten <id> [--root .] [--depth 3] [--include-type a,b] [--exclude-type a,b
 Create a compact AI context pack.
 
 ```bash
-gsp pack <id> [--root .] [--depth 3] [--budget 12000] [--out context-pack.json]
+gsp pack <id> [--root .] [--for implement] [--stage implement] [--depth 3] [--budget 12000] [--format json|md|canvas] [--out context-pack.json]
 ```
 
+The `--for` value describes the task intent:
+
+```text
+design
+implement
+review
+test
+acceptance
+handoff
+inspect
+```
+
+Formats:
+
+```text
+json      Machine-readable context pack.
+md        Human-readable and AI-readable context pack.
+canvas    Obsidian JSON Canvas for included GSP relations.
+```
+
+## impact
+
+Find direct and indirect GSPs affected by a GSP id.
+
+```bash
+gsp impact <id> [--root .] [--depth -1] [--format json|md|canvas] [--out impact.json]
+```
+
+Formats:
+
+```text
+json      Machine-readable impact result.
+md        Human-readable impact report.
+canvas    Obsidian JSON Canvas for affected relations.
+```
+
+## message
+
+Validate GSP agent communication messages.
+
+```bash
+gsp message validate <file> [--root .] [--out message-report.json]
+```
+
+Message files use YAML and reference GSP ids from the project.
+
 ## graph
 
 Generate a relation graph.
diff --git a/specs/versions/0.1/gsp.message.schema.json b/specs/versions/0.1/gsp.message.schema.json
new file mode 100644
index 0000000..04400c3
--- /dev/null
+++ b/specs/versions/0.1/gsp.message.schema.json
@@ -0,0 +1,63 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://gsp.local/schema/gsp.message.schema.json",
+  "title": "GSP Message",
+  "description": "GSP agent communication message schema.",
+  "type": "object",
+  "additionalProperties": true,
+  "required": ["gspMessageVersion", "id", "from", "to", "intent", "entry"],
+  "properties": {
+    "gspMessageVersion": {
+      "type": "string",
+      "enum": ["0.1"]
+    },
+    "id": {
+      "type": "string",
+      "minLength": 1
+    },
+    "from": {
+      "type": "string",
+      "minLength": 1
+    },
+    "to": {
+      "type": "string",
+      "minLength": 1
+    },
+    "intent": {
+      "type": "string",
+      "enum": ["design", "implement", "review", "test", "acceptance", "handoff", "inspect"]
+    },
+    "entry": {
+      "type": "string",
+      "minLength": 1
+    },
+    "stage": {
+      "type": "string",
+      "enum": ["design", "integrate", "implement", "bind", "release"]
+    },
+    "requires": {
+      "type": "array",
+      "items": {
+        "type": "string",
+        "minLength": 1
+      }
+    },
+    "contextPack": {
+      "type": "object",
+      "additionalProperties": true,
+      "properties": {
+        "mode": {
+          "type": "string",
+          "enum": ["design", "implement", "review", "test", "acceptance", "handoff", "inspect"]
+        },
+        "depth": {
+          "type": "integer"
+        },
+        "budget": {
+          "type": "integer",
+          "minimum": 0
+        }
+      }
+    }
+  }
+}
diff --git a/specs/versions/0.1/message.md b/specs/versions/0.1/message.md
new file mode 100644
index 0000000..7cb4612
--- /dev/null
+++ b/specs/versions/0.1/message.md
@@ -0,0 +1,41 @@
+# GSP Message 0.1
+
+GSP Message is a lightweight agent communication format.
+
+It references GSP ids and does not replace `.gsp` files.
+
+```yaml
+gspMessageVersion: 0.1
+id: msg.implement.lottery_page
+from: planner
+to: implementer
+intent: implement
+entry: page.lottery.main
+stage: implement
+requires:
+  - ui.button.reward_primary
+  - feedback.positive
+contextPack:
+  mode: implement
+  depth: -1
+  budget: 12000
+```
+
+Required fields:
+
+| Field | Meaning |
+|---|---|
+| `gspMessageVersion` | Message protocol version. |
+| `id` | Message id. |
+| `from` | Sender agent or module id. |
+| `to` | Receiver agent or module id. |
+| `intent` | Task intent. |
+| `entry` | Entry GSP id. |
+
+Validation:
+
+```bash
+gsp message validate message.gspmsg --root .
+```
+
+The validator checks message version, required fields, intent, stage, entry GSP, and required GSP references.
diff --git a/toolkit/README.md b/toolkit/README.md
index d233245..155e9ca 100644
--- a/toolkit/README.md
+++ b/toolkit/README.md
@@ -177,6 +177,8 @@ gsp index
 gsp trace <id>
 gsp flatten <id>
 gsp pack <id>
+gsp impact <id>
+gsp message validate <file>
 gsp graph <id>
 gsp stage-check --stage implement
 ```
@@ -190,6 +192,8 @@ go build -o ../bin/gsp ./cmd/gsp
 ../bin/gsp version
 ../bin/gsp validate --root ../examples/lottery
 ../bin/gsp flatten page.lottery.main --root ../examples/lottery --depth -1 --out ../.gsp/flattened.json
+../bin/gsp pack page.lottery.main --root ../examples/lottery --for implement --format md --out ../.gsp/context-pack.md
+../bin/gsp impact feedback.positive --root ../examples/lottery --format md --out ../.gsp/impact.md
 ../bin/gsp graph page.lottery.main --root ../examples/lottery --format mermaid --out ../.gsp/graph.mmd
 ../bin/gsp graph page.lottery.main --root ../examples/lottery --format md --out ../.gsp/graph.md
 ../bin/gsp graph page.lottery.main --root ../examples/lottery --format canvas --out ../.gsp/graph.canvas
@@ -216,7 +220,9 @@ go build -o ../bin/gsp ./cmd/gsp
 .gsp/graph.md
 .gsp/graph.canvas
 .gsp/context-pack.json
+.gsp/context-pack.md
 .gsp/flattened.json
+.gsp/impact.md
 ```
 
 ## 6. 必须考虑的问题
diff --git a/toolkit/cmd/gsp/completion.go b/toolkit/cmd/gsp/completion.go
index 30ed73b..496e055 100644
--- a/toolkit/cmd/gsp/completion.go
+++ b/toolkit/cmd/gsp/completion.go
@@ -12,6 +12,8 @@ $script:GspSubcommands = @(
   'trace',
   'flatten',
   'pack',
+  'impact',
+  'message',
   'graph',
   'stage-check',
   'help'
@@ -26,7 +28,9 @@ $script:GspFlags = @{
   'index' = @('--root', '--out')
   'trace' = @('--root', '--depth', '--out')
   'flatten' = @('--root', '--depth', '--include-type', '--exclude-type', '--out')
-  'pack' = @('--root', '--depth', '--budget', '--include-type', '--exclude-type', '--out')
+  'pack' = @('--root', '--for', '--stage', '--depth', '--budget', '--format', '--include-type', '--exclude-type', '--out')
+  'impact' = @('--root', '--depth', '--format', '--out')
+  'message' = @('validate')
   'graph' = @('--root', '--depth', '--format', '--out')
   'stage-check' = @('--stage', '--root', '--out')
 }
@@ -73,7 +77,11 @@ Register-ArgumentCompleter -Native -CommandName gsp -ScriptBlock {
 
   switch ($previous) {
     '--format' {
-      return @('json', 'mermaid', 'md', 'canvas') |
+      $formats = @('json', 'md', 'canvas')
+      if ($command -eq 'graph') {
+        $formats = @('json', 'mermaid', 'md', 'canvas')
+      }
+      return $formats |
         Where-Object { $_ -like "$wordToComplete*" } |
         ForEach-Object { New-GspCompletion $_ }
     }
@@ -87,6 +95,11 @@ Register-ArgumentCompleter -Native -CommandName gsp -ScriptBlock {
         Where-Object { $_ -like "$wordToComplete*" } |
         ForEach-Object { New-GspCompletion $_ }
     }
+    '--for' {
+      return @('design', 'implement', 'review', 'test', 'acceptance', 'handoff', 'inspect') |
+        Where-Object { $_ -like "$wordToComplete*" } |
+        ForEach-Object { New-GspCompletion $_ }
+    }
   }
 
   if ($command -eq 'completion') {
@@ -106,7 +119,13 @@ Register-ArgumentCompleter -Native -CommandName gsp -ScriptBlock {
       ForEach-Object { New-GspCompletion $_ }
   }
 
-  if (@('trace', 'flatten', 'pack', 'graph') -contains $command) {
+  if ($command -eq 'message') {
+    return @($script:GspFlags[$command]) |
+      Where-Object { $_ -like "$wordToComplete*" } |
+      ForEach-Object { New-GspCompletion $_ }
+  }
+
+  if (@('trace', 'flatten', 'pack', 'impact', 'graph') -contains $command) {
     return Get-GspIds |
       Where-Object { $_ -like "$wordToComplete*" } |
       ForEach-Object { New-GspCompletion $_ }
@@ -125,11 +144,19 @@ _gsp_completion() {
   cur="${COMP_WORDS[COMP_CWORD]}"
   prev="${COMP_WORDS[COMP_CWORD-1]}"
   cmd="${COMP_WORDS[1]}"
-  local commands="init ai-init version completion validate index trace flatten pack graph stage-check help"
+  local commands="init ai-init version completion validate index trace flatten pack impact message graph stage-check help"
   case "$prev" in
-    --format) COMPREPLY=( $(compgen -W "json mermaid md canvas" -- "$cur") ); return ;;
+    --format)
+      if [[ "$cmd" == "graph" ]]; then
+        COMPREPLY=( $(compgen -W "json mermaid md canvas" -- "$cur") )
+      else
+        COMPREPLY=( $(compgen -W "json md canvas" -- "$cur") )
+      fi
+      return
+      ;;
     --stage) COMPREPLY=( $(compgen -W "design integrate implement bind release" -- "$cur") ); return ;;
     --skill) COMPREPLY=( $(compgen -W "generic codex" -- "$cur") ); return ;;
+    --for) COMPREPLY=( $(compgen -W "design implement review test acceptance handoff inspect" -- "$cur") ); return ;;
   esac
   if [[ $COMP_CWORD -eq 1 ]]; then
     COMPREPLY=( $(compgen -W "$commands" -- "$cur") )
@@ -143,7 +170,9 @@ _gsp_completion() {
       validate|index) COMPREPLY=( $(compgen -W "--root --out" -- "$cur") ) ;;
       trace) COMPREPLY=( $(compgen -W "--root --depth --out" -- "$cur") ) ;;
       flatten) COMPREPLY=( $(compgen -W "--root --depth --include-type --exclude-type --out" -- "$cur") ) ;;
-      pack) COMPREPLY=( $(compgen -W "--root --depth --budget --include-type --exclude-type --out" -- "$cur") ) ;;
+      pack) COMPREPLY=( $(compgen -W "--root --for --stage --depth --budget --format --include-type --exclude-type --out" -- "$cur") ) ;;
+      impact) COMPREPLY=( $(compgen -W "--root --depth --format --out" -- "$cur") ) ;;
+      message) COMPREPLY=( $(compgen -W "validate" -- "$cur") ) ;;
       graph) COMPREPLY=( $(compgen -W "--root --depth --format --out" -- "$cur") ) ;;
       stage-check) COMPREPLY=( $(compgen -W "--stage --root --out" -- "$cur") ) ;;
       completion) COMPREPLY=( $(compgen -W "powershell bash zsh fish install" -- "$cur") ) ;;
@@ -151,7 +180,7 @@ _gsp_completion() {
     return
   fi
   case "$cmd" in
-    trace|flatten|pack|graph)
+    trace|flatten|pack|impact|graph)
       local ids
       ids=$(gsp index --root . 2>/dev/null | sed -n 's/.*"id": "\([^"]*\)".*/\1/p')
       COMPREPLY=( $(compgen -W "$ids" -- "$cur") )
@@ -177,6 +206,8 @@ _gsp() {
     'trace'
     'flatten'
     'pack'
+    'impact'
+    'message'
     'graph'
     'stage-check'
     'help'
@@ -189,10 +220,12 @@ _gsp "$@"
 
 func fishCompletionScript() string {
 	return `# GSP fish completion
-complete -c gsp -f -n '__fish_use_subcommand' -a 'init ai-init version completion validate index trace flatten pack graph stage-check help'
+complete -c gsp -f -n '__fish_use_subcommand' -a 'init ai-init version completion validate index trace flatten pack impact message graph stage-check help'
 complete -c gsp -n '__fish_seen_subcommand_from graph' -l format -a 'json mermaid md canvas'
+complete -c gsp -n '__fish_seen_subcommand_from pack impact' -l format -a 'json md canvas'
 complete -c gsp -n '__fish_seen_subcommand_from stage-check' -l stage -a 'design integrate implement bind release'
 complete -c gsp -n '__fish_seen_subcommand_from ai-init' -l skill -a 'generic codex'
-complete -c gsp -n '__fish_seen_subcommand_from trace flatten pack graph' -a '(gsp index --root . 2>/dev/null | string match -r ''"id": "([^"]+)"'' | string replace -r ''.*"id": "([^"]+)".*'' ''$1'')'
+complete -c gsp -n '__fish_seen_subcommand_from pack' -l for -a 'design implement review test acceptance handoff inspect'
+complete -c gsp -n '__fish_seen_subcommand_from trace flatten pack impact graph' -a '(gsp index --root . 2>/dev/null | string match -r ''"id": "([^"]+)"'' | string replace -r ''.*"id": "([^"]+)".*'' ''$1'')'
 `
 }
diff --git a/toolkit/cmd/gsp/main.go b/toolkit/cmd/gsp/main.go
index edb5db9..d570d34 100644
--- a/toolkit/cmd/gsp/main.go
+++ b/toolkit/cmd/gsp/main.go
@@ -48,6 +48,10 @@ func run(args []string) error {
 		return runFlatten(args[1:])
 	case "pack":
 		return runPack(args[1:])
+	case "impact":
+		return runImpact(args[1:])
+	case "message":
+		return runMessage(args[1:])
 	case "graph":
 		return runGraph(args[1:])
 	case "stage-check":
@@ -73,7 +77,9 @@ Usage:
   gsp index [--root .] [--out index.json]
   gsp trace <id> [--root .] [--depth 3] [--out trace.json]
   gsp flatten <id> [--root .] [--depth 3] [--include-type a,b] [--exclude-type a,b] [--out flattened.json]
-  gsp pack <id> [--root .] [--depth 3] [--budget 12000] [--out context-pack.json]
+  gsp pack <id> [--root .] [--for implement] [--stage implement] [--depth 3] [--budget 12000] [--format json|md|canvas] [--out context-pack.json]
+  gsp impact <id> [--root .] [--depth -1] [--format json|md|canvas] [--out impact.json]
+  gsp message validate <file> [--root .] [--out message-report.json]
   gsp graph [id] [--root .] [--depth 3] [--format json|mermaid|md|canvas] [--out graph.json]
   gsp stage-check --stage implement [--root .] [--out stage-report.json]
 `)
@@ -320,6 +326,9 @@ func runPack(args []string) error {
 	out := commonOut(fs)
 	depth := fs.Int("depth", 3, "maximum relation depth; -1 means unlimited")
 	budget := fs.Int("budget", 0, "approximate JSON character budget; 0 means unlimited")
+	intent := fs.String("for", "", "context purpose: design, implement, review, test, acceptance, handoff, or inspect")
+	stage := fs.String("stage", "", "optional project stage")
+	format := fs.String("format", "json", "json, md, or canvas")
 	includeType := fs.String("include-type", "", "comma-separated type allow-list")
 	excludeType := fs.String("exclude-type", "", "comma-separated type deny-list")
 	if err := fs.Parse(normalizeFlagArgs(args)); err != nil {
@@ -332,11 +341,91 @@ func runPack(args []string) error {
 	if err != nil {
 		return err
 	}
-	result := project.Pack(fs.Arg(0), *depth, *budget, gsp.Filter{
+	result := project.PackFor(fs.Arg(0), *intent, *stage, *depth, *budget, gsp.Filter{
 		IncludeTypes: splitCSV(*includeType),
 		ExcludeTypes: splitCSV(*excludeType),
 	})
-	return writeJSON(*out, result)
+	switch *format {
+	case "json":
+		return writeJSON(*out, result)
+	case "md":
+		return writeText(*out, result.Markdown())
+	case "canvas":
+		data, err := project.Graph(fs.Arg(0), *depth).Canvas()
+		if err != nil {
+			return err
+		}
+		return writeBytes(*out, data)
+	default:
+		return fmt.Errorf("unsupported pack format %q", *format)
+	}
+}
+
+func runImpact(args []string) error {
+	fs := flag.NewFlagSet("impact", flag.ContinueOnError)
+	root := commonRoot(fs)
+	out := commonOut(fs)
+	depth := fs.Int("depth", -1, "maximum reverse relation depth; -1 means unlimited")
+	format := fs.String("format", "json", "json, md, or canvas")
+	if err := fs.Parse(normalizeFlagArgs(args)); err != nil {
+		return err
+	}
+	if fs.NArg() != 1 {
+		return fmt.Errorf("impact requires one GSP id")
+	}
+	project, err := gsp.LoadProject(*root)
+	if err != nil {
+		return err
+	}
+	result := project.Impact(fs.Arg(0), *depth)
+	switch *format {
+	case "json":
+		return writeJSON(*out, result)
+	case "md":
+		return writeText(*out, result.Markdown())
+	case "canvas":
+		data, err := project.ImpactGraph(result).Canvas()
+		if err != nil {
+			return err
+		}
+		return writeBytes(*out, data)
+	default:
+		return fmt.Errorf("unsupported impact format %q", *format)
+	}
+}
+
+func runMessage(args []string) error {
+	if len(args) == 0 {
+		return fmt.Errorf("message requires validate")
+	}
+	switch args[0] {
+	case "validate":
+		return runMessageValidate(args[1:])
+	default:
+		return fmt.Errorf("unknown message command %q", args[0])
+	}
+}
+
+func runMessageValidate(args []string) error {
+	fs := flag.NewFlagSet("message validate", flag.ContinueOnError)
+	root := commonRoot(fs)
+	out := commonOut(fs)
+	if err := fs.Parse(normalizeFlagArgs(args)); err != nil {
+		return err
+	}
+	if fs.NArg() != 1 {
+		return fmt.Errorf("message validate requires one file")
+	}
+	project, err := gsp.LoadProject(*root)
+	if err != nil {
+		return err
+	}
+	message, err := gsp.ReadMessage(project.Root, fs.Arg(0))
+	if err != nil {
+		return err
+	}
+	report := project.ValidateMessage(message)
+	return writeReport(*out, report)
 }
 
 func runGraph(args []string) error {
@@ -418,6 +507,7 @@ func normalizeFlagArgs(args []string) []string {
 		"-budget": true, "--budget": true,
 		"-format": true, "--format": true,
 		"-stage": true, "--stage": true,
+		"-for": true, "--for": true,
 		"-name": true, "--name": true,
 		"-entry": true, "--entry": true,
 	}
diff --git a/toolkit/internal/gsp/ai_init.go b/toolkit/internal/gsp/ai_init.go
index 6cc765c..dec65c4 100644
--- a/toolkit/internal/gsp/ai_init.go
+++ b/toolkit/internal/gsp/ai_init.go
@@ -116,6 +116,8 @@ func aiUsage(gspVersion, scan string) string {
 - Use `+"`gsp trace <id>`"+` to inspect relations.
 - Use `+"`gsp flatten <id>`"+` before implementation or task splitting.
 - Use `+"`gsp pack <id>`"+` when a compact AI context is needed.
+- Use `+"`gsp impact <id>`"+` before changing shared GSP units.
+- Use `+"`gsp message validate <file>`"+` for agent communication messages.
 - Use `+"`gsp stage-check --stage <stage>`"+` before stage handoff.
 
 `, gspVersion, scan)
diff --git a/toolkit/internal/gsp/format.go b/toolkit/internal/gsp/format.go
new file mode 100644
index 0000000..23dccdc
--- /dev/null
+++ b/toolkit/internal/gsp/format.go
@@ -0,0 +1,113 @@
+package gsp
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+func (p PackResult) Markdown() string {
+	var builder strings.Builder
+	builder.WriteString("# GSP Context Pack\n\n")
+	builder.WriteString(fmt.Sprintf("- Entry: `%s`\n", p.Entry))
+	if p.Intent != "" {
+		builder.WriteString(fmt.Sprintf("- Intent: `%s`\n", p.Intent))
+	}
+	if p.Stage != "" {
+		builder.WriteString(fmt.Sprintf("- Stage: `%s`\n", p.Stage))
+	}
+	builder.WriteString(fmt.Sprintf("- Units: `%d`\n", p.Summary.UnitCount))
+	if p.Summary.MinResolution != "" {
+		builder.WriteString(fmt.Sprintf("- Resolution: `%s` to `%s`\n", p.Summary.MinResolution, p.Summary.MaxResolution))
+	}
+	if p.Budget > 0 {
+		builder.WriteString(fmt.Sprintf("- Budget: `%d`\n", p.Budget))
+		builder.WriteString(fmt.Sprintf("- Approx chars: `%d`\n", p.ApproxChars))
+		builder.WriteString(fmt.Sprintf("- Truncated: `%v`\n", p.Truncated))
+	}
+	builder.WriteString("\n## Units\n\n")
+	for _, unit := range p.Units {
+		builder.WriteString(fmt.Sprintf("### %s\n\n", unit.DisplayTitle()))
+		builder.WriteString(fmt.Sprintf("- id: `%s`\n", unit.ID))
+		if unit.Type != "" {
+			builder.WriteString(fmt.Sprintf("- type: `%s`\n", unit.Type))
+		}
+		if unit.Resolution != "" {
+			builder.WriteString(fmt.Sprintf("- resolution: `%s`\n", unit.Resolution))
+		}
+		if unit.Refines != "" {
+			builder.WriteString(fmt.Sprintf("- refines: `%s`\n", unit.Refines))
+		}
+		if len(unit.With) > 0 {
+			var with []string
+			for _, rel := range unit.With {
+				with = append(with, "`"+rel.ID+"`")
+			}
+			sort.Strings(with)
+			builder.WriteString("- with: " + strings.Join(with, ", ") + "\n")
+		}
+		if unit.Context != "" {
+			builder.WriteString("\n")
+			builder.WriteString(unit.Context)
+			builder.WriteString("\n")
+		}
+		builder.WriteString("\n")
+	}
+	if len(p.Warnings) > 0 {
+		builder.WriteString("## Warnings\n\n")
+		for _, warning := range p.Warnings {
+			builder.WriteString(fmt.Sprintf("- `%s`: %s\n", warning.Code, warning.Message))
+		}
+	}
+	return builder.String()
+}
+
+func (r ImpactResult) Markdown() string {
+	var builder strings.Builder
+	builder.WriteString("# GSP Impact\n\n")
+	builder.WriteString(fmt.Sprintf("- Entry: `%s`\n", r.Entry))
+	builder.WriteString(fmt.Sprintf("- Direct affected: `%d`\n", r.Summary.DirectCount))
+	builder.WriteString(fmt.Sprintf("- Indirect affected: `%d`\n", r.Summary.IndirectCount))
+	builder.WriteString(fmt.Sprintf("- Max depth: `%d`\n", r.Summary.MaxDepth))
+	writeImpactSection(&builder, "Direct", r.Direct)
+	writeImpactSection(&builder, "Indirect", r.Indirect)
+	if len(r.Warnings) > 0 {
+		builder.WriteString("## Warnings\n\n")
+		for _, warning := range r.Warnings {
+			builder.WriteString(fmt.Sprintf("- `%s`: %s\n", warning.Code, warning.Message))
+		}
+	}
+	return builder.String()
+}
+
+func writeImpactSection(builder *strings.Builder, title string, entries []ImpactEntry) {
+	builder.WriteString("\n## " + title + "\n\n")
+	if len(entries) == 0 {
+		builder.WriteString("None.\n")
+		return
+	}
+	for _, entry := range entries {
+		label := entry.Title
+		if label == "" {
+			label = entry.ID
+		}
+		builder.WriteString(fmt.Sprintf("- `%s` %s", entry.ID, label))
+		if entry.Kind != "" || entry.Via != "" {
+			builder.WriteString(fmt.Sprintf(" via `%s` `%s`", entry.Kind, entry.Via))
+		}
+		if entry.Type != "" {
+			builder.WriteString(fmt.Sprintf(" type `%s`", entry.Type))
+		}
+		if entry.Resolution != "" {
+			builder.WriteString(fmt.Sprintf(" resolution `%s`", entry.Resolution))
+		}
+		builder.WriteString("\n")
+	}
+}
+
+func (u Unit) DisplayTitle() string {
+	if strings.TrimSpace(u.Title) != "" {
+		return u.Title
+	}
+	return u.ID
+}
diff --git a/toolkit/internal/gsp/graph.go b/toolkit/internal/gsp/graph.go
index 3cd95b3..146ab36 100644
--- a/toolkit/internal/gsp/graph.go
+++ b/toolkit/internal/gsp/graph.go
@@ -96,6 +96,33 @@ func (g Graph) Markdown() string {
 	return builder.String()
 }
 
+func (p *Project) ImpactGraph(impact ImpactResult) Graph {
+	nodeMap := map[string]GraphNode{}
+	if unit, ok := p.ByID[impact.Entry]; ok {
+		nodeMap[impact.Entry] = GraphNode{ID: unit.ID, Title: unit.Title, Type: unit.Type, Resolution: unit.Resolution, File: unit.File}
+	} else {
+		nodeMap[impact.Entry] = GraphNode{ID: impact.Entry, Missing: true}
+	}
+	for _, entry := range append(append([]ImpactEntry{}, impact.Direct...), impact.Indirect...) {
+		nodeMap[entry.ID] = GraphNode{ID: entry.ID, Title: entry.Title, Type: entry.Type, Resolution: entry.Resolution, File: entry.File}
+		if _, ok := nodeMap[entry.Via]; !ok && entry.Via != "" {
+			if unit, exists := p.ByID[entry.Via]; exists {
+				nodeMap[entry.Via] = GraphNode{ID: unit.ID, Title: unit.Title, Type: unit.Type, Resolution: unit.Resolution, File: unit.File}
+			} else {
+				nodeMap[entry.Via] = GraphNode{ID: entry.Via, Missing: true}
+			}
+		}
+	}
+	nodes := make([]GraphNode, 0, len(nodeMap))
+	for _, node := range nodeMap {
+		nodes = append(nodes, node)
+	}
+	sort.Slice(nodes, func(i, j int) bool {
+		return nodes[i].ID < nodes[j].ID
+	})
+	return Graph{Nodes: nodes, Edges: impact.Edges}
+}
+
 func (g Graph) Canvas() ([]byte, error) {
 	canvas := canvasDocument{
 		Nodes: make([]canvasNode, 0, len(g.Nodes)),
diff --git a/toolkit/internal/gsp/message.go b/toolkit/internal/gsp/message.go
new file mode 100644
index 0000000..19834ff
--- /dev/null
+++ b/toolkit/internal/gsp/message.go
@@ -0,0 +1,99 @@
+package gsp
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v3"
+)
+
+const GSPMessageVersion = "0.1"
+
+var allowedMessageIntents = map[string]bool{
+	"design":     true,
+	"implement":  true,
+	"review":     true,
+	"test":       true,
+	"acceptance": true,
+	"handoff":    true,
+	"inspect":    true,
+}
+
+type Message struct {
+	MessageVersion string      `json:"gspMessageVersion,omitempty" yaml:"gspMessageVersion"`
+	ID             string      `json:"id,omitempty" yaml:"id"`
+	From           string      `json:"from,omitempty" yaml:"from"`
+	To             string      `json:"to,omitempty" yaml:"to"`
+	Intent         string      `json:"intent,omitempty" yaml:"intent"`
+	Entry          string      `json:"entry,omitempty" yaml:"entry"`
+	Stage          string      `json:"stage,omitempty" yaml:"stage"`
+	Requires       []string    `json:"requires,omitempty" yaml:"requires"`
+	ContextPack    ContextPack `json:"contextPack,omitempty" yaml:"contextPack"`
+	File           string      `json:"file,omitempty" yaml:"-"`
+}
+
+type ContextPack struct {
+	Mode   string `json:"mode,omitempty" yaml:"mode"`
+	Depth  int    `json:"depth,omitempty" yaml:"depth"`
+	Budget int    `json:"budget,omitempty" yaml:"budget"`
+}
+
+func ReadMessage(root, file string) (Message, error) {
+	path := file
+	if !filepath.IsAbs(path) {
+		path = filepath.Join(root, file)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return Message{}, err
+	}
+	var message Message
+	if err := yaml.Unmarshal(data, &message); err != nil {
+		return Message{}, err
+	}
+	message.File = relPath(root, path)
+	return message, nil
+}
+
+func (p *Project) ValidateMessage(message Message) Report {
+	report := Report{OK: true}
+	if message.MessageVersion == "" {
+		report.addError("missing_message_version", message.ID, message.File, "message requires gspMessageVersion")
+	} else if message.MessageVersion != GSPMessageVersion {
+		report.addError("unsupported_message_version", message.ID, message.File, fmt.Sprintf("GSP message version %q is not supported", message.MessageVersion))
+	}
+	if message.ID == "" {
+		report.addError("missing_id", "", message.File, "message requires id")
+	}
+	if message.From == "" {
+		report.addError("missing_from", message.ID, message.File, "message requires from")
+	}
+	if message.To == "" {
+		report.addError("missing_to", message.ID, message.File, "message requires to")
+	}
+	if message.Intent == "" {
+		report.addError("missing_intent", message.ID, message.File, "message requires intent")
+	} else if !allowedMessageIntents[message.Intent] {
+		report.addError("invalid_intent", message.ID, message.File, fmt.Sprintf("intent %q is not allowed", message.Intent))
+	}
+	if message.Entry == "" {
+		report.addError("missing_entry", message.ID, message.File, "message requires entry")
+	} else if _, ok := p.ByID[message.Entry]; !ok {
+		report.addError("missing_entry_gsp", message.Entry, message.File, fmt.Sprintf("entry references missing GSP %q", message.Entry))
+	}
+	if message.Stage != "" {
+		if _, ok := p.Manifest.minResolution(message.Stage); !ok {
+			report.addError("unknown_stage", message.ID, message.File, fmt.Sprintf("unknown stage %q", message.Stage))
+		}
+	}
+	for _, id := range message.Requires {
+		if _, ok := p.ByID[id]; !ok {
+			report.addError("missing_required_gsp", id, message.File, fmt.Sprintf("requires references missing GSP %q", id))
+		}
+	}
+	if message.ContextPack.Mode != "" && !allowedMessageIntents[message.ContextPack.Mode] {
+		report.addError("invalid_context_pack_mode", message.ID, message.File, fmt.Sprintf("contextPack mode %q is not allowed", message.ContextPack.Mode))
+	}
+	return report
+}
diff --git a/toolkit/internal/gsp/model.go b/toolkit/internal/gsp/model.go
index 0d8c22e..66a36e3 100644
--- a/toolkit/internal/gsp/model.go
+++ b/toolkit/internal/gsp/model.go
@@ -131,14 +131,53 @@ type TraceResult struct {
 
 type PackResult struct {
 	Entry       string  `json:"entry"`
+	Intent      string  `json:"intent,omitempty"`
+	Stage       string  `json:"stage,omitempty"`
 	Depth       int     `json:"depth"`
 	Budget      int     `json:"budget,omitempty"`
 	Truncated   bool    `json:"truncated"`
 	Units       []*Unit `json:"units"`
+	Summary     Summary `json:"summary"`
 	ApproxChars int     `json:"approxChars"`
 	Warnings    []Issue `json:"warnings,omitempty"`
 }
 
+type Summary struct {
+	UnitCount     int            `json:"unitCount"`
+	MinResolution string         `json:"minResolution,omitempty"`
+	MaxResolution string         `json:"maxResolution,omitempty"`
+	TypeCounts    map[string]int `json:"typeCounts,omitempty"`
+	MissingCount  int            `json:"missingCount,omitempty"`
+}
+
+type ImpactResult struct {
+	Entry    string        `json:"entry"`
+	Depth    int           `json:"depth"`
+	Summary  ImpactSummary `json:"summary"`
+	Direct   []ImpactEntry `json:"direct,omitempty"`
+	Indirect []ImpactEntry `json:"indirect,omitempty"`
+	Edges    []GraphEdge   `json:"edges,omitempty"`
+	Warnings []Issue       `json:"warnings,omitempty"`
+}
+
+type ImpactSummary struct {
+	DirectCount   int `json:"directCount"`
+	IndirectCount int `json:"indirectCount"`
+	TotalCount    int `json:"totalCount"`
+	MaxDepth      int `json:"maxDepth"`
+}
+
+type ImpactEntry struct {
+	ID         string `json:"id"`
+	Title      string `json:"title,omitempty"`
+	Type       string `json:"type,omitempty"`
+	Resolution string `json:"resolution,omitempty"`
+	File       string `json:"file,omitempty"`
+	Depth      int    `json:"depth"`
+	Via        string `json:"via,omitempty"`
+	Kind       string `json:"kind,omitempty"`
+}
+
 type Graph struct {
 	Nodes []GraphNode `json:"nodes"`
 	Edges []GraphEdge `json:"edges"`
diff --git a/toolkit/internal/gsp/project.go b/toolkit/internal/gsp/project.go
index ae9f3a1..1777428 100644
--- a/toolkit/internal/gsp/project.go
+++ b/toolkit/internal/gsp/project.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"sort"
+	"strings"
 )
 
 func (p *Project) Validate(stage string) Report {
@@ -165,6 +166,10 @@ func (p *Project) Flatten(id string, depth int, filter Filter) FlattenResult {
 }
 
 func (p *Project) Pack(id string, depth, budget int, filter Filter) PackResult {
+	return p.PackFor(id, "", "", depth, budget, filter)
+}
+
+func (p *Project) PackFor(id, intent, stage string, depth, budget int, filter Filter) PackResult {
 	flattened := p.Flatten(id, depth, filter)
 	units := make([]*Unit, 0, len(flattened.Units))
 	approx := 0
@@ -181,15 +186,131 @@ func (p *Project) Pack(id string, depth, budget int, filter Filter) PackResult {
 	}
 	return PackResult{
 		Entry:       id,
+		Intent:      intent,
+		Stage:       stage,
 		Depth:       depth,
 		Budget:      budget,
 		Truncated:   truncated,
 		Units:       units,
+		Summary:     summarizeUnits(units, len(flattened.Warnings)),
 		ApproxChars: approx,
 		Warnings:    flattened.Warnings,
 	}
 }
 
+func summarizeUnits(units []*Unit, missingCount int) Summary {
+	summary := Summary{
+		UnitCount:    len(units),
+		TypeCounts:   map[string]int{},
+		MissingCount: missingCount,
+	}
+	min := 99
+	max := -1
+	for _, unit := range units {
+		if unit.Type != "" {
+			summary.TypeCounts[unit.Type]++
+		}
+		rank, ok := resolutionValue(unit.Resolution)
+		if !ok {
+			continue
+		}
+		if rank < min {
+			min = rank
+			summary.MinResolution = displayResolution(unit.Resolution)
+		}
+		if rank > max {
+			max = rank
+			summary.MaxResolution = displayResolution(unit.Resolution)
+		}
+	}
+	if len(summary.TypeCounts) == 0 {
+		summary.TypeCounts = nil
+	}
+	return summary
+}
+
+func (p *Project) Impact(id string, depth int) ImpactResult {
+	result := ImpactResult{Entry: id, Depth: depth}
+	if _, ok := p.ByID[id]; !ok {
+		result.Warnings = append(result.Warnings, Issue{Level: "warning", Code: "missing", ID: id, Message: fmt.Sprintf("missing GSP %q", id)})
+		return result
+	}
+
+	reverse := map[string][]ImpactEntry{}
+	for _, unit := range p.Units {
+		if unit.Refines != "" {
+			reverse[unit.Refines] = append(reverse[unit.Refines], impactEntry(unit, 0, unit.Refines, "refines"))
+		}
+		for _, rel := range unit.With {
+			reverse[rel.ID] = append(reverse[rel.ID], impactEntry(unit, 0, rel.ID, "with"))
+		}
+	}
+	for key := range reverse {
+		sort.Slice(reverse[key], func(i, j int) bool {
+			return reverse[key][i].ID < reverse[key][j].ID
+		})
+	}
+
+	seen := map[string]bool{id: true}
+	queue := []ImpactEntry{{ID: id, Depth: 0}}
+	for len(queue) > 0 {
+		current := queue[0]
+		queue = queue[1:]
+		if depth >= 0 && current.Depth >= depth {
+			continue
+		}
+		for _, affected := range reverse[current.ID] {
+			if seen[affected.ID] {
+				continue
+			}
+			seen[affected.ID] = true
+			affected.Depth = current.Depth + 1
+			affected.Via = current.ID
+			if affected.Depth == 1 {
+				result.Direct = append(result.Direct, affected)
+			} else {
+				result.Indirect = append(result.Indirect, affected)
+			}
+			result.Edges = append(result.Edges, GraphEdge{From: affected.ID, To: current.ID, Kind: affected.Kind})
+			queue = append(queue, affected)
+			if affected.Depth > result.Summary.MaxDepth {
+				result.Summary.MaxDepth = affected.Depth
+			}
+		}
+	}
+	sortImpactEntries(result.Direct)
+	sortImpactEntries(result.Indirect)
+	sort.Slice(result.Edges, func(i, j int) bool {
+		return strings.Join([]string{result.Edges[i].From, result.Edges[i].To, result.Edges[i].Kind}, "\x00") < strings.Join([]string{result.Edges[j].From, result.Edges[j].To, result.Edges[j].Kind}, "\x00")
+	})
+	result.Summary.DirectCount = len(result.Direct)
+	result.Summary.IndirectCount = len(result.Indirect)
+	result.Summary.TotalCount = len(result.Direct) + len(result.Indirect)
+	return result
+}
+
+func impactEntry(unit *Unit, depth int, via, kind string) ImpactEntry {
+	return ImpactEntry{
+		ID:         unit.ID,
+		Title:      unit.Title,
+		Type:       unit.Type,
+		Resolution: unit.Resolution,
+		File:       unit.File,
+		Depth:      depth,
+		Via:        via,
+		Kind:       kind,
+	}
+}
+
+func sortImpactEntries(entries []ImpactEntry) {
+	sort.Slice(entries, func(i, j int) bool {
+		if entries[i].Depth == entries[j].Depth {
+			return entries[i].ID < entries[j].ID
+		}
+		return entries[i].Depth < entries[j].Depth
+	})
+}
+
 type walker struct {
 	project  *Project
 	depth    int
diff --git a/toolkit/internal/gsp/project_test.go b/toolkit/internal/gsp/project_test.go
index 34b2636..aa0c25b 100644
--- a/toolkit/internal/gsp/project_test.go
+++ b/toolkit/internal/gsp/project_test.go
@@ -91,6 +91,25 @@ context: Base lottery page.
 	if !foundTitle {
 		t.Fatalf("expected canvas node text to include title, got %+v", canvas.Nodes)
 	}
+
+	pack := project.PackFor("page.lottery.main", "implement", "implement", -1, 0, Filter{})
+	if pack.Summary.UnitCount != 4 {
+		t.Fatalf("pack unit count = %d", pack.Summary.UnitCount)
+	}
+	if !strings.Contains(pack.Markdown(), "GSP Context Pack") {
+		t.Fatal("expected pack markdown")
+	}
+
+	impact := project.Impact("feedback.positive", -1)
+	if impact.Summary.DirectCount != 1 {
+		t.Fatalf("impact direct count = %d", impact.Summary.DirectCount)
+	}
+	if impact.Direct[0].ID != "page.lottery.main" {
+		t.Fatalf("impact direct = %+v", impact.Direct)
+	}
+	if !strings.Contains(impact.Markdown(), "GSP Impact") {
+		t.Fatal("expected impact markdown")
+	}
 }
 
 func TestValidateMissingReference(t *testing.T) {
@@ -263,6 +282,40 @@ func TestInitAIUsageRequiresManifest(t *testing.T) {
 	}
 }
 
+func TestValidateMessage(t *testing.T) {
+	root := t.TempDir()
+	if _, err := InitProject(root, InitOptions{
+		Entry: "page.sample.main",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	writeTestFile(t, root, "message.gspmsg", `gspMessageVersion: 0.1
+id: msg.sample
+from: planner
+to: implementer
+intent: implement
+entry: page.sample.main
+stage: implement
+requires:
+  - page.sample.main
+contextPack:
+  mode: implement
+  depth: -1
+`)
+	project, err := LoadProject(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	message, err := ReadMessage(project.Root, "message.gspmsg")
+	if err != nil {
+		t.Fatal(err)
+	}
+	report := project.ValidateMessage(message)
+	if !report.OK {
+		t.Fatalf("expected valid message, got %+v", report.Errors)
+	}
+}
+
 func writeTestFile(t *testing.T, root, name, content string) {
 	t.Helper()
 	path := filepath.Join(root, name)