diff --git a/.opencode/plugins/ecc-hooks.ts b/.opencode/plugins/ecc-hooks.ts
index 4aabde612..bf06c03f8 100644
--- a/.opencode/plugins/ecc-hooks.ts
+++ b/.opencode/plugins/ecc-hooks.ts
@@ -481,7 +481,7 @@ export const ECCHooksPlugin: ECCHooksPluginFn = async ({
      * Triggers: Before shell command execution
      * Action: Sets PROJECT_ROOT, PACKAGE_MANAGER, DETECTED_LANGUAGES, ECC_VERSION
      */
-    "shell.env": async () => {
+    "shell.env": async (_input: { cwd: string }, output: { env: Record<string, string> }) => {
       const env: Record<string, string> = {
         ECC_VERSION: getECCVersion(),
         ECC_PLUGIN: "true",
@@ -523,7 +523,8 @@ export const ECCHooksPlugin: ECCHooksPluginFn = async ({
         env.PRIMARY_LANGUAGE = detected[0]
       }
 
-      return env
+      // OpenCode reads the supplied output object and ignores callback return values.
+      output.env = { ...output.env, ...env }
     },
 
     /**
@@ -531,9 +532,12 @@ export const ECCHooksPlugin: ECCHooksPluginFn = async ({
      * OpenCode-specific: Control context compaction behavior
      *
      * Triggers: Before context compaction
-     * Action: Push ECC context block and custom compaction prompt
+     * Action: Push ECC context block and compaction guidance
      */
-    "experimental.session.compacting": async () => {
+    "experimental.session.compacting": async (
+      _input: { sessionID: string },
+      output: { context: string[]; prompt?: string }
+    ) => {
       const contextBlock = [
         "# ECC Context (preserve across compaction)",
         "",
@@ -558,9 +562,16 @@ export const ECCHooksPlugin: ECCHooksPluginFn = async ({
         contextBlock.push("")
       }
 
-      return {
-        context: contextBlock.join("\n"),
-        compaction_prompt: "Focus on preserving: 1) Current task status and progress, 2) Key decisions made, 3) Files created/modified, 4) Remaining work items, 5) Any security concerns flagged. Discard: verbose tool outputs, intermediate exploration, redundant file listings.",
+      const eccContext = [
+        contextBlock.join("\n"),
+        "Focus on preserving: 1) Current task status and progress, 2) Key decisions made, 3) Files created/modified, 4) Remaining work items, 5) Any security concerns flagged. Discard: verbose tool outputs, intermediate exploration, redundant file listings.",
+      ]
+
+      // OpenCode requires output assignment and skips context when a prompt is set.
+      if (output.prompt !== undefined) {
+        output.prompt = [output.prompt, ...eccContext].join("\n\n")
+      } else {
+        output.context = [...output.context, ...eccContext]
       }
     },
 
diff --git a/tests/opencode-plugin-hooks.test.js b/tests/opencode-plugin-hooks.test.js
index 0d261ef5c..a6c3ed2ae 100644
--- a/tests/opencode-plugin-hooks.test.js
+++ b/tests/opencode-plugin-hooks.test.js
@@ -179,9 +179,14 @@ async function main() {
           const $ = createFailingShell()
           const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
 
-          const env = await hooks["shell.env"]()
+          const existingEnv = Object.freeze({ EXISTING_ENV: "preserved" })
+          const output = { env: existingEnv }
+          await hooks["shell.env"]({ cwd: projectDir }, output)
+          const { env } = output
 
           assert.deepStrictEqual($.calls, [], `Unexpected shell probes: ${$.calls.join(", ")}`)
+          assert.strictEqual(env.EXISTING_ENV, "preserved")
+          assert.notStrictEqual(env, existingEnv)
           assert.strictEqual(env.PROJECT_ROOT, projectDir)
           assert.strictEqual(env.PACKAGE_MANAGER, "pnpm")
           assert.strictEqual(env.DETECTED_LANGUAGES, "typescript,python")
@@ -243,9 +248,12 @@ async function main() {
           const $ = createFailingShell()
           const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
 
-          const env = await hooks["shell.env"]()
+          const output = { env: {} }
+          await hooks["shell.env"]({ cwd: projectDir }, output)
+          const { env } = output
 
           assert.deepStrictEqual($.calls, [], `Unexpected shell probes: ${$.calls.join(", ")}`)
+          assert.strictEqual(env.PROJECT_ROOT, projectDir)
           assert.ok(!("PACKAGE_MANAGER" in env), "Lockfile directory should not set PACKAGE_MANAGER")
           assert.ok(!("DETECTED_LANGUAGES" in env), "Marker directory should not set DETECTED_LANGUAGES")
           assert.ok(!("PRIMARY_LANGUAGE" in env), "Marker directory should not set PRIMARY_LANGUAGE")
@@ -254,6 +262,47 @@ async function main() {
         }
       },
     ],
+    [
+      "compacting appends ECC context without replacing the host compaction prompt",
+      async () => withTempProject([], async (projectDir) => {
+        const client = createClient()
+        const $ = createFailingShell()
+        const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
+        const existingContext = Object.freeze(["Existing plugin context"])
+        const output = { context: existingContext }
+
+        await hooks["experimental.session.compacting"]({ sessionID: "session-1" }, output)
+
+        assert.strictEqual(output.context[0], "Existing plugin context")
+        assert.notStrictEqual(output.context, existingContext)
+        const prompt = output.prompt ?? ["Default compaction prompt", ...output.context].join("\n\n")
+        assert.ok(prompt.includes("Default compaction prompt"))
+        assert.ok(prompt.includes("# ECC Context"))
+        assert.ok(prompt.includes("Current task status and progress"))
+        assert.deepStrictEqual($.calls, [])
+      }),
+    ],
+    [
+      "compacting appends ECC guidance to custom prompts, including an empty prompt",
+      async () => withTempProject([], async (projectDir) => {
+        const client = createClient()
+        const $ = createFailingShell()
+        const hooks = await ECCHooksPlugin({ client, $, directory: projectDir })
+
+        for (const customPrompt of ["Another plugin's custom prompt", ""]) {
+          const existingContext = Object.freeze(["Existing plugin context"])
+          const output = { context: existingContext, prompt: customPrompt }
+          await hooks["experimental.session.compacting"]({ sessionID: "session-1" }, output)
+
+          const prompt = output.prompt ?? ["Default compaction prompt", ...output.context].join("\n\n")
+          assert.ok(prompt.startsWith(`${customPrompt}\n\n`))
+          assert.ok(prompt.includes("# ECC Context"))
+          assert.ok(prompt.includes("Current task status and progress"))
+          assert.strictEqual(output.context, existingContext)
+        }
+        assert.deepStrictEqual($.calls, [])
+      }),
+    ],
     [
       "permission.ask handles read-only tools correctly",
       async () => withTempProject(