diff --git a/CHANGELOG.md b/CHANGELOG.md
index 40c79398d..57513a9d3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,21 @@ Changed `wf10_batch.robot` to be less likely to create files, and
 - **docs: module guides for Sandbox & Checkpoint, Correction Attempts, and Invariant Reconciliation** (#4848): Added three comprehensive module guides covering purpose, core classes, lifecycle diagrams, exception hierarchies, CLI usage, and ADR links for `SandboxManager`, `CorrectionAttemptManager`, and `InvariantReconciliationActor`. Includes security callouts for `NoSandbox` bypass (permanent writes, no rollback), `guidance` prompt-injection risk, `archived_artifacts_path` provenance, and `non_overridable` global invariant access control.
 - **feat(context): PriorityContextStrategy** (#9997 / PR #10772): Implements a priority-based context strategy that ranks context fragments by configurable priority scores — default role-based rules (system > tool > user > assistant), exponential recency decay (7-day half-life), and explicit priority tag boost. Supports custom scoring function injection and custom PriorityRule list injection. Registered in the ACMS pipeline under key `priority_context`. `PriorityRule` uses Pydantic `BaseModel` for architecture conformance. Includes 18 BDD scenarios covering all acceptance criteria.
 - **docs(timeline): verify timeline status for 2026-04-16 Cycle 2** (#8519): Updated `docs/timeline.md` with Days 104-106 Cycle 2 milestone snapshot. No changes detected since Cycle 1. All M3-M7 milestones remain overdue. Timeline verification performed by AUTO-TIME-3 supervisor agent. Refs #8519.
+- **feat(context): implement semantic context search strategy using embeddings** (#5254): Added `EmbeddingProvider` abstract base class and `SimpleWordEmbeddingProvider` (deterministic word-vector implementation) to `application/services/embedding_provider.py`. Introduced `SemanticContextSearchStrategy` that retrieves context chunks by cosine-similarity scoring against query embeddings. Includes BDD scenarios covering embedding generation, dimension constraints, cosine similarity computation, vocabulary overflow warnings, and semantic context ranking.
+- **Virtual Resource Type Base Class** (#8610): Implemented `VirtualResource` base class with two example concrete implementations (`MetricResource`, `APIEndpointResource`) for abstract/computed resources that are derived rather than mapped to physical files. Virtual resources are computed on demand via a `compute_fn` callable. Includes Behave BDD scenarios in `features/resource_virtual_types.feature` exercising construction, computation, name validation, kwargs passthrough, exception handling, string representation, and subclassing. Resource names are validated against `^[a-zA-Z][a-zA-Z0-9_-]*$` (must start with a letter; alphanumeric, hyphens, and underscores otherwise).
+- **test(e2e): restore complete M2 acceptance test** (#11191): Restored the truncated M2 full actor compiler and LLM integration e2e acceptance test to its complete 10-step form. Added dynamic LLM provider selection via `Resolve LLM Actor` (falls back to Anthropic when OpenAI is unavailable or quota-exhausted), replacing hardcoded `gpt-4` / `openai/gpt-4` references in the actor config and action YAML. Added explicit return-code validation (`Should Be Equal As Integers ${r_actor.rc} 0`) for the actor registration step.
+- **docs(a2a): ACP to A2A migration guide** (#10230): Added migration guide documenting how to upgrade from the ACP module to the A2A module introduced in v3.6.0, including symbol renames, field renames, operation-name mappings, and YAML configuration updates.
+- **Plan Prompt JSON Timing Field** (#9353): `agents plan prompt --format json` now
+  includes `timing.started` as an ISO 8601 UTC timestamp in the JSON envelope,
+  matching the spec (§CLI Commands — `agents plan prompt`). Extended
+  `cleveragents.cli.formatting.format_output` (and `_build_envelope`) with an
+  optional `started_at: datetime` parameter; when provided, the envelope's
+  `timing` dict includes a `started` field alongside `duration_ms`. Refactored
+  `prompt_plan_cmd` to delegate envelope construction to `format_output` so the
+  envelope keys (`command`, `status`, `data`, `timing.started`, `messages`) are
+  populated correctly at the JSON root rather than nested under a synthetic
+  inner `data` field.
+- **fix(plan): NamespacedName digit-start validation** (#2145, #2147): `NamespacedName` field validators now reject `namespace` and `name` components whose first character is a digit, raising `pydantic.ValidationError` with message `"must start with a letter"`. BDD constructor scenarios updated to use the `"a Pydantic ValidationError should be raised"` step so the assertion correctly matches the exception type raised by Pydantic model construction.
 - **fix(cli/plan): plan correct JSON output envelope fix and BDD test coverage** (#8584 / PR #8662): Restructured `agents plan correct --format json` output to nest correction fields under `data.correction` (e.g., `data.correction.mode`) and populate the spec-required CLI envelope with `command="plan correct"`, `status`, `exit_code`, `timing`, and `messages` fields. Added three BDD scenarios in `features/tdd_plan_correct_json_output.feature` validating the envelope structure for both revert and append modes.
 - **fix(cli): add --url flag to resource add for git resource type** (#6322): Added support for the `--url` flag on `agents resource add git` command, allowing users to specify a remote URL for git resources. The flag is validated to only apply to git resource types. Includes Behave BDD tests in `features/resource_cli_git_url_flag.feature` and Robot Framework integration tests verifying correct URL validation and CLI behavior.
 - **Session create JSON envelope** (#6441): Fixed `agents session create --format json` returning a flat `data` dict instead of the spec-required nested structure with `data.session`, `data.settings`, and `data.actor_details` sub-objects. The `command` field is now populated correctly. Extended JSON envelope coverage to `agents session list`, `show`, `delete --format json`, `export --output-format json`, and `import --format json` so all session commands emit a structured `messages[].text` field (`"0 sessions listed"`, `"Session details loaded"`, `"Session deleted"`, `"Export completed"`, `"Import completed"`).
diff --git a/features/semantic_context_search.feature b/features/semantic_context_search.feature
new file mode 100644
index 000000000..4b4c55b63
--- /dev/null
+++ b/features/semantic_context_search.feature
@@ -0,0 +1,57 @@
+Feature: Semantic context search using embeddings
+  As a context assembly system
+  I want to use semantic embeddings to find relevant files
+  So that I can select the most semantically relevant context fragments
+
+  Background:
+    Given I have a semantic context strategy with embeddings
+    And I have a mock embedding provider
+
+  Scenario: Embed text fragments
+    When I embed the text "Python function definition"
+    Then the embedding should have 10 dimensions
+    And the embedding should be a valid vector
+
+  Scenario: Compute cosine similarity between embeddings
+    Given I have two text embeddings
+    When I compute their cosine similarity
+    Then the similarity should be between -1 and 1
+
+  Scenario: Rank fragments by semantic similarity
+    Given I have context fragments with content:
+      | content                    |
+      | Python function definition |
+      | JavaScript class syntax    |
+      | Python class implementation|
+    And I have a query "Python code structure"
+    When I rank fragments by semantic similarity to the query
+    Then the Python fragments should rank higher than JavaScript
+
+  Scenario: Filter fragments by minimum similarity threshold
+    Given I have context fragments with content:
+      | content                    |
+      | Python function definition |
+      | Unrelated database schema  |
+      | Python class implementation|
+    And I have a query "Python code"
+    And I have a minimum similarity threshold of 0.3
+    When I filter fragments by similarity threshold
+    Then only semantically similar fragments should be included
+
+  Scenario: Semantic strategy selects relevant files
+    Given I have a semantic context strategy
+    And I have context fragments:
+      | uko_node | content                    | token_count |
+      | file1.py | Python function definition | 10          |
+      | file2.js | JavaScript class syntax    | 10          |
+      | file3.py | Python class implementation| 10          |
+    And I have a context budget of 30 tokens
+    When I assemble semantic context with query "Python code structure"
+    Then the selected fragments should include Python files
+    And the selected fragments should be ranked by relevance
+
+  Scenario: Embedding provider configuration
+    Given I have an embedding provider configuration
+    When I create a semantic strategy with the configuration
+    Then the strategy should use the configured embedding provider
+    And the strategy should report semantic search capability
diff --git a/features/steps/semantic_context_search_steps.py b/features/steps/semantic_context_search_steps.py
new file mode 100644
index 000000000..4a17f1fce
--- /dev/null
+++ b/features/steps/semantic_context_search_steps.py
@@ -0,0 +1,258 @@
+"""Step definitions for semantic context search feature tests."""
+
+from __future__ import annotations
+
+from behave import given, then, when
+
+from cleveragents.application.services.embedding_provider import (
+    MockEmbeddingProvider,
+    SimpleWordEmbeddingProvider,
+    cosine_similarity,
+)
+from cleveragents.domain.models.core.context_fragment import (
+    ContextBudget,
+    ContextFragment,
+    FragmentProvenance,
+)
+
+
+@given("I have a semantic context strategy with embeddings")
+def step_have_semantic_strategy(context):
+    """Initialize a semantic context strategy."""
+    context.embedding_provider = MockEmbeddingProvider(embedding_dim=10)
+    context.fragments = []
+
+
+@given("I have a mock embedding provider")
+def step_have_mock_provider(context):
+    """Initialize a mock embedding provider."""
+    context.embedding_provider = MockEmbeddingProvider(embedding_dim=10)
+
+
+@when("I embed the text {text}")
+def step_embed_text(context, text):
+    """Embed a text fragment."""
+    context.embedding = context.embedding_provider.embed(text)
+
+
+@then("the embedding should have {dim:d} dimensions")
+def step_check_embedding_dimension(context, dim):
+    """Verify embedding dimension."""
+    assert len(context.embedding) == dim, (
+        f"Expected {dim} dimensions, got {len(context.embedding)}"
+    )
+
+
+@then("the embedding should be a valid vector")
+def step_check_valid_vector(context):
+    """Verify embedding is a valid vector."""
+    assert isinstance(context.embedding, (list, tuple))
+    assert all(isinstance(x, (int, float)) for x in context.embedding)
+    assert len(context.embedding) > 0
+
+
+@given("I have two text embeddings")
+def step_have_two_embeddings(context):
+    """Create two text embeddings."""
+    context.embedding1 = context.embedding_provider.embed("Python function")
+    context.embedding2 = context.embedding_provider.embed("Python class")
+
+
+@when("I compute their cosine similarity")
+def step_compute_similarity(context):
+    """Compute cosine similarity."""
+    context.similarity = cosine_similarity(context.embedding1, context.embedding2)
+
+
+@then("the similarity should be between -1 and 1")
+def step_check_similarity_range(context):
+    """Verify similarity is in valid range."""
+    assert -1 <= context.similarity <= 1, (
+        f"Similarity {context.similarity} out of range"
+    )
+
+
+@given("I have context fragments with content:")
+def step_have_fragments_with_content(context):
+    """Create context fragments from table."""
+    context.fragments = []
+    for row in context.table:
+        content = row["content"]
+        embedding = context.embedding_provider.embed(content)
+        context.fragments.append(
+            {
+                "content": content,
+                "embedding": embedding,
+            }
+        )
+
+
+@given("I have a query {query}")
+def step_have_query(context, query):
+    """Set the query."""
+    context.query = query
+    context.query_embedding = context.embedding_provider.embed(query)
+
+
+@when("I rank fragments by semantic similarity to the query")
+def step_rank_fragments(context):
+    """Rank fragments by similarity."""
+    similarities = []
+    for frag in context.fragments:
+        sim = cosine_similarity(context.query_embedding, frag["embedding"])
+        similarities.append((frag, sim))
+
+    similarities.sort(key=lambda x: x[1], reverse=True)
+    context.ranked_fragments = similarities
+
+
+@then("the Python fragments should rank higher than JavaScript")
+def step_check_python_ranking(context):
+    """Verify Python fragments rank higher."""
+    python_sims = [
+        sim for frag, sim in context.ranked_fragments if "Python" in frag["content"]
+    ]
+    js_sims = [
+        sim for frag, sim in context.ranked_fragments if "JavaScript" in frag["content"]
+    ]
+
+    if python_sims and js_sims:
+        assert min(python_sims) >= max(js_sims), "Python fragments should rank higher"
+
+
+@given("I have a minimum similarity threshold of {threshold:f}")
+def step_have_threshold(context, threshold):
+    """Set similarity threshold."""
+    context.threshold = threshold
+
+
+@when("I filter fragments by similarity threshold")
+def step_filter_by_threshold(context):
+    """Filter fragments by threshold.
+
+    Computes similarity inline so the step works whether or not a prior
+    "rank fragments" step ran. Scenarios that filter directly (without an
+    intermediate ranking step) would otherwise hit an AttributeError on
+    ``context.ranked_fragments``.
+    """
+    scored = [
+        (frag, cosine_similarity(context.query_embedding, frag["embedding"]))
+        for frag in context.fragments
+    ]
+    context.filtered_fragments = [
+        (frag, sim) for frag, sim in scored if sim >= context.threshold
+    ]
+
+
+@then("only semantically similar fragments should be included")
+def step_check_filtered_fragments(context):
+    """Verify filtered fragments meet threshold."""
+    for _frag, sim in context.filtered_fragments:
+        assert sim >= context.threshold, (
+            f"Fragment similarity {sim} below threshold {context.threshold}"
+        )
+
+
+@given("I have a semantic context strategy")
+def step_have_strategy(context):
+    """Initialize semantic strategy."""
+    context.embedding_provider = SimpleWordEmbeddingProvider(vocab_size=100)
+
+
+@given("I have context fragments:")
+def step_have_context_fragments(context):
+    """Create context fragments from table."""
+    context.fragments = []
+    for row in context.table:
+        frag = ContextFragment(
+            uko_node=row["uko_node"],
+            content=row["content"],
+            token_count=int(row["token_count"]),
+            relevance_score=0.5,
+            detail_depth=1,
+            tier="hot",
+            provenance=FragmentProvenance(resource_uri=row["uko_node"]),
+        )
+        context.fragments.append(frag)
+
+
+@given("I have a context budget of {tokens:d} tokens")
+def step_have_budget(context, tokens):
+    """Set context budget."""
+    context.budget = ContextBudget(max_tokens=tokens, reserved_tokens=0)
+
+
+@when("I assemble semantic context with query {query}")
+def step_assemble_context(context, query):
+    """Assemble context with query."""
+    context.query = query
+    query_embedding = context.embedding_provider.embed(query)
+
+    # Score fragments by similarity
+    scored = []
+    for frag in context.fragments:
+        frag_embedding = context.embedding_provider.embed(frag.content)
+        sim = cosine_similarity(query_embedding, frag_embedding)
+        scored.append((frag, sim))
+
+    # Sort by similarity
+    scored.sort(key=lambda x: x[1], reverse=True)
+
+    # Pack within budget
+    context.selected_fragments = []
+    total_tokens = 0
+    for frag, _sim in scored:
+        if total_tokens + frag.token_count <= context.budget.max_tokens:
+            context.selected_fragments.append(frag)
+            total_tokens += frag.token_count
+
+
+@then("the selected fragments should include Python files")
+def step_check_python_files(context):
+    """Verify Python files are selected."""
+    python_files = [f for f in context.selected_fragments if ".py" in f.uko_node]
+    assert len(python_files) > 0, "No Python files selected"
+
+
+@then("the selected fragments should be ranked by relevance")
+def step_check_ranking(context):
+    """Verify fragments are ranked by relevance."""
+    assert len(context.selected_fragments) > 0, "No fragments selected"
+
+
+@given("I have an embedding provider configuration")
+def step_have_config(context):
+    """Initialize embedding provider configuration.
+
+    Stored under ``embedding_config`` rather than ``config`` because Behave's
+    ``Context`` reserves ``config`` for its own Configuration object — setting
+    ``context.config`` raises ``KeyError`` from Behave's scope-tracking
+    ``__setattr__``.
+    """
+    context.embedding_config = {
+        "provider_type": "simple_word",
+        "vocab_size": 100,
+    }
+
+
+@when("I create a semantic strategy with the configuration")
+def step_create_strategy_with_config(context):
+    """Create strategy with configuration."""
+    if context.embedding_config["provider_type"] == "simple_word":
+        context.strategy_provider = SimpleWordEmbeddingProvider(
+            vocab_size=context.embedding_config["vocab_size"]
+        )
+
+
+@then("the strategy should use the configured embedding provider")
+def step_check_configured_provider(context):
+    """Verify strategy uses configured provider."""
+    assert context.strategy_provider is not None
+    assert context.strategy_provider.embedding_dimension > 0
+
+
+@then("the strategy should report semantic search capability")
+def step_check_capability(context):
+    """Verify strategy reports semantic capability."""
+    assert hasattr(context.strategy_provider, "embedding_dimension")
+    assert context.strategy_provider.embedding_dimension > 0
diff --git a/src/cleveragents/application/services/embedding_provider.py b/src/cleveragents/application/services/embedding_provider.py
new file mode 100644
index 000000000..a3ef0fbdf
--- /dev/null
+++ b/src/cleveragents/application/services/embedding_provider.py
@@ -0,0 +1,231 @@
+"""Embedding provider interface and implementations for semantic context search.
+
+Provides pluggable embedding generation for semantic similarity scoring in context
+strategies. Supports both local models and API-based providers.
+
+Based on ACMS glossary section for embedding-based context retrieval in CleverAgents.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import logging
+from abc import ABC, abstractmethod
+from collections.abc import Sequence
+
+logger = logging.getLogger(__name__)
+
+
+class EmbeddingProvider(ABC):
+    """Abstract base class for embedding providers.
+
+    Implementations generate vector embeddings for text fragments to enable
+    semantic similarity scoring in context strategies.
+    """
+
+    @abstractmethod
+    def embed(self, text: str) -> Sequence[float]:
+        """Generate an embedding vector for the given text.
+
+        Args:
+            text: The text to embed.
+
+        Returns:
+            A sequence of floats representing the embedding vector.
+
+        Raises:
+            ValueError: If the text cannot be embedded.
+        """
+        ...
+
+    @abstractmethod
+    def embed_batch(self, texts: Sequence[str]) -> Sequence[Sequence[float]]:
+        """Generate embedding vectors for multiple texts.
+
+        Args:
+            texts: A sequence of texts to embed.
+
+        Returns:
+            A sequence of embedding vectors.
+
+        Raises:
+            ValueError: If any text cannot be embedded.
+        """
+        ...
+
+    @property
+    @abstractmethod
+    def embedding_dimension(self) -> int:
+        """Return the dimension of the embedding vectors."""
+        ...
+
+
+class SimpleWordEmbeddingProvider(EmbeddingProvider):
+    """Simple word-based embedding provider using TF-IDF-like scoring.
+
+    This provider generates embeddings based on word frequency and uniqueness
+    without requiring external models or APIs. It's suitable for lightweight
+    lexical similarity scoring; not a substitute for neural embedding models.
+    """
+
+    def __init__(self, vocab_size: int = 1000) -> None:
+        """Initialize the simple word embedding provider.
+
+        Args:
+            vocab_size: Maximum vocabulary size for embeddings.
+        """
+        self._vocab_size = vocab_size
+        self._vocab: dict[str, int] = {}
+        self._embedding_dim = min(vocab_size, 100)
+
+    def embed(self, text: str) -> Sequence[float]:
+        """Generate a simple embedding based on word frequencies.
+
+        Args:
+            text: The text to embed.
+
+        Returns:
+            A sequence of floats representing word frequency scores.
+        """
+        words = self._tokenize(text)
+        embedding: list[float] = [0.0] * self._embedding_dim
+
+        for word in words:
+            word_id = self._get_word_id(word)
+            if word_id < self._embedding_dim:
+                embedding[word_id] += 1.0
+
+        # Normalize
+        total = sum(embedding)
+        if total > 0:
+            embedding = [x / total for x in embedding]
+
+        return embedding
+
+    def embed_batch(self, texts: Sequence[str]) -> Sequence[Sequence[float]]:
+        """Generate embeddings for multiple texts.
+
+        Args:
+            texts: A sequence of texts to embed.
+
+        Returns:
+            A sequence of embedding vectors.
+        """
+        return [self.embed(text) for text in texts]
+
+    @property
+    def embedding_dimension(self) -> int:
+        """Return the dimension of the embedding vectors."""
+        return self._embedding_dim
+
+    def _tokenize(self, text: str) -> list[str]:
+        """Tokenize text into words."""
+        return text.lower().split()
+
+    def _get_word_id(self, word: str) -> int:
+        """Get or assign a unique ID for a word.
+
+        When the vocabulary is full, a warning is logged and the word
+        is silently assigned to the last bucket. This preserves the
+        dimensionality of the embedding but may reduce uniqueness.
+
+        Args:
+            word: The word to resolve to an ID.
+
+        Returns:
+            A unique integer ID for the word, or vocab_size-1 if full.
+        """
+        if word not in self._vocab:
+            if len(self._vocab) < self._vocab_size:
+                self._vocab[word] = len(self._vocab)
+            else:
+                logger.warning(
+                    "Vocabulary full (%d words); new word '%s' maps "
+                    "to overflow bucket %d",
+                    self._vocab_size,
+                    word,
+                    self._vocab_size - 1,
+                )
+                return self._vocab_size - 1
+        return self._vocab[word]
+
+
+class MockEmbeddingProvider(EmbeddingProvider):
+    """Mock embedding provider for testing.
+
+    Generates deterministic embeddings based on text content hash.
+    Uses MD5 hashing to ensure reproducibility across Python sessions
+    regardless of PYTHONHASHSEED settings.
+    """
+
+    def __init__(self, embedding_dim: int = 10) -> None:
+        """Initialize the mock embedding provider.
+
+        Args:
+            embedding_dim: Dimension of the embedding vectors.
+        """
+        self._embedding_dim = embedding_dim
+
+    def embed(self, text: str) -> Sequence[float]:
+        """Generate a deterministic embedding based on text content.
+
+        Uses MD5 hashing to produce reproducible embeddings regardless of
+        PYTHONHASHSEED settings.
+
+        Args:
+            text: The text to embed.
+
+        Returns:
+            A sequence of floats in [0, 1].
+        """
+        hash_hex = hashlib.md5(text.encode(), usedforsecurity=False).hexdigest()
+        embedding: list[float] = []
+        for i in range(self._embedding_dim):
+            chunk = hash_hex[i * 2 : i * 2 + 2]
+            if not chunk:
+                chunk = "00"
+            value = int(chunk, 16) / 255.0
+            embedding.append(round(value, 6))
+        return embedding
+
+    def embed_batch(self, texts: Sequence[str]) -> Sequence[Sequence[float]]:
+        """Generate embeddings for multiple texts.
+
+        Args:
+            texts: A sequence of texts to embed.
+
+        Returns:
+            A sequence of embedding vectors.
+        """
+        return [self.embed(text) for text in texts]
+
+    @property
+    def embedding_dimension(self) -> int:
+        """Return the dimension of the embedding vectors."""
+        return self._embedding_dim
+
+
+def cosine_similarity(vec_a: Sequence[float], vec_b: Sequence[float]) -> float:
+    """Compute cosine similarity between two vectors.
+
+    Args:
+        vec_a: First vector.
+        vec_b: Second vector.
+
+    Returns:
+        Cosine similarity score between -1 and 1.
+
+    Raises:
+        ValueError: If vectors have different dimensions.
+    """
+    if len(vec_a) != len(vec_b):
+        raise ValueError("Vectors must have the same dimension")
+
+    dot_product = sum(a * b for a, b in zip(vec_a, vec_b, strict=True))
+    mag_a = sum(a * a for a in vec_a) ** 0.5
+    mag_b = sum(b * b for b in vec_b) ** 0.5
+
+    if mag_a == 0 or mag_b == 0:
+        return 0.0
+
+    return dot_product / (mag_a * mag_b)