@@ -22,7 +22,10 @@ export const Plugin = define({
     const hook = (event: SessionHooks["context"]) =>
       Effect.gen(function* () {
         const model = yield* OptimizePlugin.resolveModel(ctx, event)
-        event.system.splice(1, 0, SystemPart.make(identity({ name: model.name, ref: event.model })))
+        event.system.splice(1, 0, {
+          ...SystemPart.make(identity({ name: model.name, ref: event.model })),
+          metadata: { experimental: { key: "core/identity" } },
+        })
       }).pipe(Effect.catch(() => Effect.void))
     yield* ctx.session.hook("context", hook)
     yield* ctx.session.hook("compaction", hook)
 
@@ -8,14 +8,15 @@ import { ClaudeProvider } from "./provider.js"
 
 const KINDS = ["context", "generate", "compaction"] as const
 
-const InstructionsMetadata = Schema.Struct({
+const PartMetadata = Schema.Struct({
   experimental: Schema.Struct({
-    instructions: Schema.Array(Schema.Struct({ key: Schema.String, text: Schema.String })),
+    key: Schema.optional(Schema.String),
+    instructions: Schema.optional(Schema.Array(Schema.Struct({ key: Schema.String, text: Schema.String }))),
   }),
 })
 
-// Claude Code writes its own date and environment, so OpenCode's would repeat them.
-const SUPPLIED_BY_CLAUDE = new Set(["core/date", "core/environment"])
+// Claude Code writes its own base prompt, model identity, date and environment, so OpenCode's would repeat them.
+const SUPPLIED_BY_CLAUDE = new Set(["core/prompt", "core/identity", "core/date", "core/environment"])
 
 export const register = Effect.fn("ClaudeInstructions.register")(function* (ctx: Context) {
   yield* Effect.forEach(
@@ -51,19 +52,15 @@ export const register = Effect.fn("ClaudeInstructions.register")(function* (ctx:
   )
 })
 
-// The first part is OpenCode's own prompt, which Claude Code's prompt replaces.
+// OpenCode tags the parts Claude Code supplies itself, so plugins may add parts anywhere in the array.
 function instructions(system: ReadonlyArray<SystemPart>) {
-  const texts: string[] = []
-  for (const part of system.slice(1)) {
-    const metadata = Schema.decodeUnknownOption(InstructionsMetadata)(part.metadata)
-    if (Option.isNone(metadata)) {
-      texts.push(part.text)
-      continue
-    }
-    for (const source of metadata.value.experimental.instructions) {
-      if (SUPPLIED_BY_CLAUDE.has(source.key)) continue
-      texts.push(source.text)
-    }
-  }
-  return texts.join("\n\n")
+  return system
+    .flatMap((part) => {
+      const metadata = Option.getOrUndefined(Schema.decodeUnknownOption(PartMetadata)(part.metadata))?.experimental
+      if (metadata?.key && SUPPLIED_BY_CLAUDE.has(metadata.key)) return []
+      if (!metadata?.instructions) return [part.text]
+      return metadata.instructions.filter((source) => !SUPPLIED_BY_CLAUDE.has(source.key)).map((source) => source.text)
+    })
+    .filter(Boolean)
+    .join("\n\n")
 }
@@ -109,11 +109,15 @@ export const baseTranscript = (input: {
   return {
     providerMetadataKey,
     system: [
-      SystemPart.make(
-        input.agent.system
-          ? input.agent.system
-          : SessionSystemPrompt.make(input.tools.definitions.map((tool) => tool.name)),
-      ),
+      {
+        ...SystemPart.make(
+          input.agent.system
+            ? input.agent.system
+            : SessionSystemPrompt.make(input.tools.definitions.map((tool) => tool.name)),
+        ),
+        // Lets providers with their own base prompt, such as Claude Code, find and replace this one.
+        metadata: { experimental: { key: "core/prompt" } },
+      },
       {
         ...SystemPart.make(Instructions.join(input.initial)),
         metadata: { experimental: { instructions: input.initial } },
 
@@ -414,7 +414,7 @@ describe("ClaudeAgentPlugin session hooks", () => {
       const hooks = yield* PluginHooks.Service
       const event = hookEvent((yield* newSession).id)
       event.system = [
-        SystemPart.make("OPENCODE-PROMPT"),
+        { ...SystemPart.make("OPENCODE-PROMPT"), metadata: { experimental: { key: "core/prompt" } } },
         {
           ...SystemPart.make("JOINED-INSTRUCTIONS"),
           metadata: {
@@ -457,6 +457,32 @@ describe("ClaudeAgentPlugin session hooks", () => {
     }),
   )
 
+  // Claude Code writes its own base prompt and model identity, so OpenCode's tagged parts are left out by tag,
+  // wherever plugins have moved them, and parts a plugin adds at the front are kept.
+  it.live("skips the parts Claude Code supplies by tag, not by position", () =>
+    Effect.gen(function* () {
+      yield* addPlugin(FakeIO.make({ claude: FakeClaude.make([]) }))
+      yield* addAgents
+      const hooks = yield* PluginHooks.Service
+      yield* hooks.register("session", "context", (event) =>
+        Effect.sync(() => {
+          event.system.unshift(SystemPart.make("FIRST-PLUGIN-PART"))
+          event.system.push(SystemPart.make(""), SystemPart.make("LAST-PLUGIN-PART"))
+        }),
+      )
+      const event = hookEvent((yield* newSession).id)
+      event.system = [
+        { ...SystemPart.make("OPENCODE-PROMPT"), metadata: { experimental: { key: "core/prompt" } } },
+        { ...SystemPart.make("IDENTITY"), metadata: { experimental: { key: "core/identity" } } },
+        SystemPart.make("RULES"),
+      ]
+      const context = yield* hooks.trigger("session", "context", event)
+      expect(context.options[ClaudeSystemPrompt.INSTRUCTIONS_OPTION]).toBe(
+        "FIRST-PLUGIN-PART\n\nRULES\n\nLAST-PLUGIN-PART",
+      )
+    }),
+  )
+
   it.live("lets a later plugin override the instructions outright", () =>
     Effect.gen(function* () {
       yield* addPlugin(FakeIO.make({ claude: FakeClaude.make([]) }))